Improve PEP 383 tests (in test_os)

 * Use the current filesystem encoding instead of always using utf-8
 * Enable the test on Mac OS X
 * Use TESTFN_UNENCODABLE and TESTFN_UNICODE instead of arbitrary filenames
 * To decode a filename, use strict error handler instead surrogateescape for
   mbcs encoding (on Windows)
 * Use TESTFN_UNENCODABLE (if available) for the directory name

Skip the test if no non-ascii filename can be created.
diff --git a/Lib/test/support.py b/Lib/test/support.py
index 8a8e410..a70fee4 100644
--- a/Lib/test/support.py
+++ b/Lib/test/support.py
@@ -377,10 +377,8 @@
 TESTFN = "{}_{}_tmp".format(TESTFN, os.getpid())
 
 
-# Assuming sys.getfilesystemencoding()!=sys.getdefaultencoding()
-# TESTFN_UNICODE is a filename that can be encoded using the
-# file system encoding, but *not* with the default (ascii) encoding
-TESTFN_UNICODE = TESTFN + "-\xe0\xf2"
+# TESTFN_UNICODE is a non-ascii filename
+TESTFN_UNICODE = TESTFN + "-\xe0\xf2\u0258\u0141\u011f"
 TESTFN_ENCODING = sys.getfilesystemencoding()
 
 # TESTFN_UNENCODABLE is a filename (str type) that should *not* be able to be