Fix return_value of Mock in test

tests/server_notices/test_resource_limits_server_notices.py

@@ -88,7 +88,7 @@ class TestResourceLimitsServerNotices(unittest.HomeserverTestCase):
     def test_maybe_send_server_notice_to_user_remove_blocked_notice(self):
         """Test when user has blocked notice, but should have it removed"""
 
-        self._rlsn._auth.check_auth_blocking = Mock(defer.succeed(None))
+        self._rlsn._auth.check_auth_blocking = Mock(return_value=defer.succeed(None))
         mock_event = Mock(
             type=EventTypes.Message, content={"msgtype": ServerNoticeMsgType}
         )