Issue #16414: Add support.FS_NONASCII and support.TESTFN_NONASCII

These constants are used to test functions with non-ASCII data, especially
filenames.
diff --git a/Lib/test/test_genericpath.py b/Lib/test/test_genericpath.py
index 600ffc0..b1ee110 100644
--- a/Lib/test/test_genericpath.py
+++ b/Lib/test/test_genericpath.py
@@ -313,6 +313,8 @@
     def test_nonascii_abspath(self):
         if support.TESTFN_UNDECODABLE:
             name = support.TESTFN_UNDECODABLE
+        elif support.TESTFN_NONASCII:
+            name = support.TESTFN_NONASCII
         else:
             name = b'a\xffb\xe7w\xf0'