Fix tests on non-English Windows (#12878)
By default, unlike on CI, a Windows machine does not allow creating
symlinks, so we have to unix-gate tests that assume symlinks.
We can't install the transformers ecosystem test on Windows due to
missing torch, so it is also unix-gated.
Windows translates error messages, so we have to filter the "File not
found" message, since it can also be a "Datei nicht gefunden".
