mentions: Send user group mention data to notification notices.

We will later use this data to include text like:
`<sender> mentioned @<user_group>` instead of the current
`<sender> mentioned you` when someone mentions a user group
the current user is a part of in email/push notification.

Part of #13080.

zerver/lib/test_classes.py

@@ -1355,6 +1355,7 @@ Output:
             acting_user_id=acting_user_id,
             private_message=kwargs.get("private_message", False),
             stream_name=kwargs.get("stream_name", None),
+            mentioned_user_group_id=kwargs.get("mentioned_user_group_id", None),
             idle=kwargs.get("idle", True),
             already_notified=kwargs.get(
                 "already_notified", {"email_notified": False, "push_notified": False}