bpo-33709: test_ntpath and test_posixpath fail in Windows with ACP!=1252. (GH-7278)

Lib/test/support/__init__.py

@@ -871,7 +871,11 @@ for character in (
     '\u20AC',
 ):
     try:
-        os.fsdecode(os.fsencode(character))
+        # If Python is set up to use the legacy 'mbcs' in Windows,
+        # 'replace' error mode is used, and encode() returns b'?'
+        # for characters missing in the ANSI codepage
+        if os.fsdecode(os.fsencode(character)) != character:
+            raise UnicodeError
     except UnicodeError:
         pass
     else: