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)
diff --git a/Lib/test/support.py b/Lib/test/support.py
index 4e946ab..801ecf2 100644
--- a/Lib/test/support.py
+++ b/Lib/test/support.py
@@ -677,7 +677,11 @@
 # decoded from the filesystem encoding (in strict mode). It can be None if we
 # cannot generate such filename.
 TESTFN_UNDECODABLE = None
-for name in (b'abc\xff', b'\xe7w\xf0'):
+# b'\xff' is not decodable by os.fsdecode() with code page 932. Windows
+# accepts it to create a file or a directory, or don't accept to enter to
+# such directory (when the bytes name is used). So test b'\xe7' first: it is
+# not decodable from cp932.
+for name in (b'\xe7w\xf0', b'abc\xff'):
     try:
         os.fsdecode(name)
     except UnicodeDecodeError: