Don't use top level actor in PriorityDispatcherSpec, see #2745

* The actor was a top level, i.e. RepointableActorRef. Messages  sent to
  self from constructor could be queued in UnstartedCell, and then sent
  to the real PriorityQueue later, processed immediately without the
  ordering taking place first.

akka-actor-tests/src/test/scala/akka/dispatch/PriorityDispatcherSpec.scala

@@ -51,19 +51,28 @@ class PriorityDispatcherSpec extends AkkaSpec(PriorityDispatcherSpec.config) wit
   def testOrdering(dispatcherKey: String) {
     val msgs = (1 to 100) toList
 
+    // 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.
     val actor = system.actorOf(Props(new Actor {
+      context.actorOf(Props(new Actor {
 
-      val acc = scala.collection.mutable.ListBuffer[Int]()
+        val acc = scala.collection.mutable.ListBuffer[Int]()
 
-      scala.util.Random.shuffle(msgs) foreach { m ⇒ self ! m }
+        scala.util.Random.shuffle(msgs) foreach { m ⇒ self ! m }
 
-      self.tell('Result, testActor)
+        self.tell('Result, testActor)
 
-      def receive = {
-        case i: Int  ⇒ acc += i
-        case 'Result ⇒ sender ! acc.toList
-      }
-    }).withDispatcher(dispatcherKey))
+        def receive = {
+          case i: Int  ⇒ acc += i
+          case 'Result ⇒ sender ! acc.toList
+        }
+      }).withDispatcher(dispatcherKey))
+
+      def receive = Actor.emptyBehavior
+
+    }))
 
     expectMsgType[List[_]] must be === msgs
   }