Fix third-party event modules for check_visibility_can_be_modified check

PR #8292 tried to maintain backwards compat with modules which don't provide a
`check_visibility_can_be_modified` method, but the tests weren't being run,
and the check didn't work.

tests/rest/client/test_third_party_rules.py

@@ -49,7 +49,7 @@ class ThirdPartyRulesTestCase(unittest.HomeserverTestCase):
     def make_homeserver(self, reactor, clock):
         config = self.default_config()
         config["third_party_event_rules"] = {
-            "module": "tests.rest.client.third_party_rules.ThirdPartyRulesTestModule",
+            "module": __name__ + ".ThirdPartyRulesTestModule",
             "config": {},
         }