Issue #16414: Fix support.TESTFN_UNDECODABLE and test_genericpath.test_nonascii_abspath()

* support.TESTFN_UNDECODABLE was decodable if the filesystem encoding was
   cp932
 * test_genericpath.test_nonascii_abspath() didn't work on Windows if the
   path was not decodable (ex: with cp932)