Huh?...
The cost of tcltest::fileEncoding will be approximately 50µs per file.
What by the 200 files amounts to "burdening" 10 milliseconds.

As for the "asciifying" in [03cf62003a88a5f2] - it is not quite kosher.

Firstly, ab\u7266 is not the same as ab牦 (first is unicode string, another is object with utf-8 bytes), what may change the test, internally.
Still worse it may look by replacement for ü - \xFC, which is single-byte, and may work for windows with cp1252 as system encoding (and unix with utf-8 by fallback from ascii through cp1252), but may fail for platforms with another single-byte system encodings, not to mention the backport to Tcls where it behaves similar to 8.5 (where \xFC may be bytearray).

Then maybe better something like [encoding convertfrom cp1252 \xFC].

However, if one see the costs... is the effort, difference to 8.7/9.0 (and a removal of, imho, nice feature) justified?