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.