Same issue exists with tests from unixSelect.test, which fail for the same reason as follows:

<verbatim>
==== unixSelect-1.1 TkSelGetSelection procedure: simple i18n text FAILED
==== Contents of test case:

    pack [entry .e]
    update
    .e insert 0 \u00fcber
    .e selection range 0 end
    dobg {string length [selection get]}

---- Result was:
PRIMARY selection doesn't exist or form "STRING" not defined
---- Result should have been (exact matching):
4
==== unixSelect-1.1 FAILED


==== unixSelect-1.2 TkSelGetSelection procedure: simple i18n text, iso8859-1 FAILED
==== Contents of test case:

    dobg {
        pack [entry .e]
        update
        .e insert 0
        .e selection range 0 end
    }
    selection get

---- Test generated error; Return code was: 1
---- Return code should have been one of: 0 2
---- errorInfo: PRIMARY selection doesn't exist or form "STRING" not defined
    while executing
"selection get"
    ("uplevel" body line 8)
    invoked from within
"uplevel 1 $script"
---- errorCode: TK SELECTION EXISTS
==== unixSelect-1.2 FAILED


==== unixSelect-1.3 TkSelGetSelection procedure: simple i18n text, iso2022 FAILED
==== Contents of test case:

    selection handle -type COMPOUND_TEXT -format COMPOUND_TEXT .  {handler COMPOUND_TEXT}
    selection own .
    set selValue
    set selInfo {}
    set result [dobg {
        set x [selection get -type COMPOUND_TEXT]
        list [string equal $x] [string length $x]
    }]
    lappend result $selInfo

---- Result was:
PRIMARY selection doesn't exist or form COMPOUND_TEXT not defined {}
---- Result should have been (exact matching):
1 2 {COMPOUND_TEXT 0 4000}
==== unixSelect-1.3 FAILED


==== unixSelect-1.4 TkSelGetSelection procedure: INCR i18n text, iso2022 FAILED
==== Contents of test case:

    # This test is subtle.  The selection ends up getting fetched twice by
    # Tk:  once to compute the length, and again to actually send the data.
    # The first time through, we don't convert the data to ISO2022, so the
    # buffer boundaries end up being different in the two passes.
    selection handle -type COMPOUND_TEXT -format COMPOUND_TEXT .  {handler COMPOUND_TEXT}
    selection own .
    set selValue [string repeat x repeat x 3999]
    set selInfo {}
    set result [dobg {
        set x [selection get -type COMPOUND_TEXT]
        list [string equal  [string repeat x repeat x 3999] $x]  [string length $x]
    }]
    lappend result $selInfo

---- Result was:
PRIMARY selection doesn't exist or form COMPOUND_TEXT not defined {}
---- Result should have been (exact matching):
1 8000 {COMPOUND_TEXT 0 4000 COMPOUND_TEXT 4000 3999 COMPOUND_TEXT 7998 4000 COMPOUND_TEXT 0 4000 COMPOUND_TEXT 4000 3998 COMPOUND_TEXT 7997 4000}
==== unixSelect-1.4 FAILED


==== unixSelect-1.5 TkSelGetSelection procedure: simple i18n text, iso2022 FAILED
==== Contents of test case:

    selection handle -type COMPOUND_TEXT -format COMPOUND_TEXT .  {handler COMPOUND_TEXT}
    selection own .
    set selValue
    set selInfo {}
    set result [dobg {
        set x [selection get -type COMPOUND_TEXT]
        list [string equal $x] [string length $x]
    }]
    lappend result $selInfo

---- Result was:
PRIMARY selection doesn't exist or form COMPOUND_TEXT not defined {}
---- Result should have been (exact matching):
1 2 {COMPOUND_TEXT 0 4000}
==== unixSelect-1.5 FAILED


==== unixSelect-1.6 TkSelGetSelection procedure: INCR i18n text FAILED
==== Contents of test case:

    dobg [subst -nobackslashes {entry .e; pack .e; update
    .e insert 0 \u00fcber$longValue
    .e selection range 0 end}]
    string length [selection get]

---- Test generated error; Return code was: 1
---- Return code should have been one of: 0 2
---- errorInfo: PRIMARY selection doesn't exist or form "STRING" not defined
    while executing
"selection get"
    ("uplevel" body line 5)
    invoked from within
"uplevel 1 $script"
---- errorCode: NONE
==== unixSelect-1.6 FAILED


==== unixSelect-1.7 TkSelGetSelection procedure: INCR i18n text FAILED
==== Contents of test case:

    dobg {
        pack [entry .e]
        update
        .e insert 0 [string repeat x 3999]\u00fc
        .e selection range 0 end
    }
    selection get

---- Test generated error; Return code was: 1
---- Return code should have been one of: 0 2
---- errorInfo: PRIMARY selection doesn't exist or form "STRING" not defined
    while executing
"selection get"
    ("uplevel" body line 8)
    invoked from within
"uplevel 1 $script"
---- errorCode: NONE
==== unixSelect-1.7 FAILED


==== unixSelect-1.8 TkSelGetSelection procedure: INCR i18n text FAILED
==== Contents of test case:

    dobg {
        pack [entry .e]
        update
        .e insert 0 \u00fc[string repeat x 3999]
        .e selection range 0 end
    }
    selection get

---- Test generated error; Return code was: 1
---- Return code should have been one of: 0 2
---- errorInfo: PRIMARY selection doesn't exist or form "STRING" not defined
    while executing
"selection get"
    ("uplevel" body line 8)
    invoked from within
"uplevel 1 $script"
---- errorCode: NONE
==== unixSelect-1.8 FAILED


==== unixSelect-1.9 TkSelGetSelection procedure: INCR i18n text FAILED
==== Contents of test case:

    dobg {
        pack [entry .e]
        update
        .e insert 0 [string repeat x 3999]\u00fc[string repeat x 4000]
        .e selection range 0 end
    }
    selection get

---- Test generated error; Return code was: 1
---- Return code should have been one of: 0 2
---- errorInfo: PRIMARY selection doesn't exist or form "STRING" not defined
    while executing
"selection get"
    ("uplevel" body line 8)
    invoked from within
"uplevel 1 $script"
---- errorCode: NONE
==== unixSelect-1.9 FAILED


==== unixSelect-1.10 TkSelGetSelection procedure: INCR i18n text, utf-8 FAILED
==== Contents of test case:

    dobg {
        pack [entry .e]
        update
        .e insert 0 [string repeat x 3999]\u00fc
        .e selection range 0 end
    }
    selection get -type UTF8_STRING

---- Test generated error; Return code was: 1
---- Return code should have been one of: 0 2
---- errorInfo: PRIMARY selection doesn't exist or form "UTF8_STRING" not defined
    while executing
"selection get -type UTF8_STRING"
    ("uplevel" body line 8)
    invoked from within
"uplevel 1 $script"
---- errorCode: NONE
==== unixSelect-1.10 FAILED


==== unixSelect-1.11 TkSelGetSelection procedure: INCR i18n text, utf-8 FAILED
==== Contents of test case:

    dobg {
        pack [entry .e]
        update
        .e insert 0 \u00fc[string repeat x 3999]
        .e selection range 0 end
    }
    selection get -type UTF8_STRING

---- Test generated error; Return code was: 1
---- Return code should have been one of: 0 2
---- errorInfo: PRIMARY selection doesn't exist or form "UTF8_STRING" not defined
    while executing
"selection get -type UTF8_STRING"
    ("uplevel" body line 8)
    invoked from within
"uplevel 1 $script"
---- errorCode: NONE
==== unixSelect-1.11 FAILED


==== unixSelect-1.12 TkSelGetSelection procedure: INCR i18n text, utf-8 FAILED
==== Contents of test case:

    dobg {
        pack [entry .e]
        update
        .e insert 0 [string repeat x 3999]\u00fc[string repeat x 4000]
        .e selection range 0 end
    }
    selection get -type UTF8_STRING

---- Test generated error; Return code was: 1
---- Return code should have been one of: 0 2
---- errorInfo: PRIMARY selection doesn't exist or form "UTF8_STRING" not defined
    while executing
"selection get -type UTF8_STRING"
    ("uplevel" body line 8)
    invoked from within
"uplevel 1 $script"
---- errorCode: NONE
==== unixSelect-1.12 FAILED


==== unixSelect-1.13 TkSelGetSelection procedure: simple i18n text, utf-8 FAILED
==== Contents of test case:

    pack [entry .e]
    update
    .e insert 0
    .e selection range 0 end
    dobg {string length [selection get -type UTF8_STRING]}

---- Result was:
PRIMARY selection doesn't exist or form "UTF8_STRING" not defined
---- Result should have been (exact matching):
5
==== unixSelect-1.13 FAILED


==== unixSelect-1.14 TkSelGetSelection procedure: simple i18n text, utf-8 FAILED
==== Contents of test case:

    dobg {
        pack [entry .e]
        update
        .e insert 0
        .e selection range 0 end
    }
    selection get -type UTF8_STRING

---- Test generated error; Return code was: 1
---- Return code should have been one of: 0 2
---- errorInfo: PRIMARY selection doesn't exist or form "UTF8_STRING" not defined
    while executing
"selection get -type UTF8_STRING"
    ("uplevel" body line 8)
    invoked from within
"uplevel 1 $script"
---- errorCode: TK SELECTION EXISTS
==== unixSelect-1.14 FAILED


==== unixSelect-1.15 TkSelGetSelection procedure: INCR i18n text, utf-8 FAILED
==== Contents of test case:

    dobg {
        pack [entry .e]
        update
        .e insert 0 [string repeat [string repeat 50]\n 21]
        .e selection range 0 end
    }
    selection get -type UTF8_STRING

---- Test generated error; Return code was: 1
---- Return code should have been one of: 0 2
---- errorInfo: PRIMARY selection doesn't exist or form "UTF8_STRING" not defined
    while executing
"selection get -type UTF8_STRING"
    ("uplevel" body line 8)
    invoked from within
"uplevel 1 $script"
---- errorCode: TK SELECTION EXISTS
==== unixSelect-1.15 FAILED


==== unixSelect-1.16 TkSelGetSelection procedure: INCR i18n text, utf-8 FAILED
==== Contents of test case:

    dobg {
        pack [entry .e]
        update
        .e insert 0 i[string repeat [string repeat 50]\n 21]
        .e selection range 0 end
    }
    selection get -type UTF8_STRING

---- Test generated error; Return code was: 1
---- Return code should have been one of: 0 2
---- errorInfo: PRIMARY selection doesn't exist or form "UTF8_STRING" not defined
    while executing
"selection get -type UTF8_STRING"
    ("uplevel" body line 8)
    invoked from within
"uplevel 1 $script"
---- errorCode: TK SELECTION EXISTS
==== unixSelect-1.16 FAILED


==== unixSelect-1.17 TkSelGetSelection procedure: INCR i18n text, utf-8 FAILED
==== Contents of test case:

    dobg {
        pack [text .t]
        update
        .t insert 1.0 [string repeat [string repeat 50]\n 21]
        # Has to be selected in a separate stage
        .t tag add sel 1.0 21.end+1c
    }
    after 10
    selection get -type UTF8_STRING

---- Test generated error; Return code was: 1
---- Return code should have been one of: 0 2
---- errorInfo: PRIMARY selection doesn't exist or form "UTF8_STRING" not defined
    while executing
"selection get -type UTF8_STRING"
    ("uplevel" body line 10)
    invoked from within
"uplevel 1 $script"
---- errorCode: TK SELECTION EXISTS
==== unixSelect-1.17 FAILED


==== unixSelect-1.18 TkSelGetSelection procedure: INCR i18n text, utf-8 FAILED
==== Contents of test case:

    dobg {
        pack [text .t]
        update
        .t insert 1.0 i[string repeat [string repeat 50]\n 21]
        # Has to be selected in a separate stage
        .t tag add sel 1.0 21.end+1c
    }
    after 10
    selection get -type UTF8_STRING

---- Test generated error; Return code was: 1
---- Return code should have been one of: 0 2
---- errorInfo: PRIMARY selection doesn't exist or form "UTF8_STRING" not defined
    while executing
"selection get -type UTF8_STRING"
    ("uplevel" body line 10)
    invoked from within
"uplevel 1 $script"
---- errorCode: TK SELECTION EXISTS
==== unixSelect-1.18 FAILED
</verbatim>

These tests need to be constrained by "x11", not by "unix". This is [fdb21d745c]