test_subs: Enforce invite_only argument to be named.

This commit enforces invite_only argument to be named
in _test_user_settings_for_creating_streams. This will
help in improving readability especially when we will
add is_web_public argument in further commits.

zerver/tests/test_subs.py

@@ -3257,7 +3257,7 @@ class SubscriptionAPITest(ZulipTestCase):
         )
 
     def _test_user_settings_for_creating_streams(
-        self, stream_policy: str, invite_only: bool
+        self, stream_policy: str, *, invite_only: bool
     ) -> None:
         user_profile = self.example_user("cordelia")
         realm = user_profile.realm