Attached patch changes how support.TESTFN_UNDECODABLE is computed on UNIX: use the filesystem encoding in *strict* mode, not using the surrogateescape error handler.
So we can use support.TESTFN_UNDECODABLE to check if a function uses correctly the surrogateescape error handler and/or check if it behaves correctly with non-ASCII characters.
The patch uses also support.TESTFN_UNDECODABLE (only on UNIX) in test_cmd_line_script.test_non_ascii() to also check that the fix for #16218 works with UTF-8 locale encoding.
Please test the patch on UNIX, Windows and Mac OS X.
We may also use support.TESTFN_UNDECODABLE in test_cmd_line_script.test_non_ascii() on Windows, I will check.
Windows has some strange behaviour with undecodable characters: some of them are replaced a character with a similar glyph.