`PriorityDispatcherSpec` has a nice comment explaining actor inside actor thing
```
    // It's important that the actor under test is not a top level actor
    // with RepointableActorRef, since messages might be queued in
    // UnstartedCell and the sent to the PriorityQueue and consumed immediately
    // without the ordering taking place.
```
it probably will be good to repeat it here as well