gh-96189: Fix test_invalid_utf8 on a number of build bots (GH-96190)

The clearing of the temporary directory is not working on some platforms and
leaving behind files.

This has been updated to use the pattern in test_cmd_line.py [1] using the
special TESTFN rather than a test directory.

[1] https://github.com/python/cpython/blob/main/Lib/test/test_cmd_line.py#L559