bpo-37828: Fix default mock_name in unittest.mock.assert_called error (GH-16166)

bpo-37828: Fix default mock_name in unittest.mock.assert_called error (GH-16166)

In the format string for assert_called the evaluation order is incorrect and hence for mock's without name, 'None' is printed whereas it should be 'mock' like for other messages. The error message is ("Expected '%s' to have been called." % self._mock_name or 'mock').