Yes, I were testing on a Linux machine and forgot that results are OS depending.
I agree, that test should less depend on implementation details. As far as _locale._getdefaultlocale is defined only on Windows and "UTF-8" is not valid locale on Windows, I think there is no need to patch _locale for testing. But getlocale() and getpreferredencoding() should be consistent with getdefaultlocale() (and getlocale() is yet one way to test private function _parse_localename()). setlocale() should work with the result of getlocale() and getdefaultlocale(). Are following tests passed on OSX?