Modernize unit tests configuration settings for workers. (#14568)

Use the newer foo_instances configuration instead of the
deprecated flags to enable specific features (e.g. start_pushers).

tests/handlers/test_presence.py

@@ -992,7 +992,8 @@ class PresenceJoinTestCase(unittest.HomeserverTestCase):
 
     def default_config(self):
         config = super().default_config()
-        config["send_federation"] = True
+        # Enable federation sending on the main process.
+        config["federation_sender_instances"] = None
         return config
 
     def prepare(self, reactor, clock, hs):