Don't encode unicode dirname in test_support.temp_cwd() if unicode file names

are supported by the filesystem.  On Windows the encoding can convert some
characters to '?' that is not legal in file name.

Lib/test/test_support.py

@@ -705,7 +705,8 @@ def temp_cwd(name='tempcwd', quiet=False):
     the CWD, an error is raised.  If it's True, only a warning is raised
     and the original CWD is used.
     """
-    if have_unicode and isinstance(name, unicode):
+    if (have_unicode and isinstance(name, unicode) and
+        not os.path.supports_unicode_filenames):
         try:
             name = name.encode(sys.getfilesystemencoding() or 'ascii')
         except UnicodeEncodeError: