Add back tests

Not exactly sure why, but it seems like I was able to get tests to pass by moving the "EventAtPath message type" describe block above "#processCallbacksFor" describe block. 

If left in the way it was before, it seems like the callbacks get mixed up (the EventAtPath block tests end up getting callbacks from the processCallbacksFor block). Not entirely sure why because we should be creating new actors every time.

Maybe we're leaking scope somehow...