Add events in Pubsub

To ensure `handle_peer_queue` and `handle_dead_peer_queue` are indeed
run before the tests finish. Previously, we get errors when performing
`iter_dag` after cancellation. This is because `handle_peer_queue` or
`handle_dead_peer_queue` is not actually run before the Service is
cancelled.

tests/pubsub/test_gossipsub.py

@@ -106,6 +106,7 @@ async def test_handle_graft(monkeypatch):
 
         async def emit_prune(topic, sender_peer_id):
             event_emit_prune.set()
+            await trio.hazmat.checkpoint()
 
         monkeypatch.setattr(gossipsubs[index_bob], "emit_prune", emit_prune)