make Jenkins wait for Davy Jones

- AkkaSpecSpec failed with nothing in the locker, but only on Jenkins
- cause: Davy Jones doesn’t get a chance to run because nobody actually
  waits for his processing
- thus, give him permission to leave this world and have him confirm
  that

akka-testkit/src/test/scala/akka/testkit/AkkaSpec.scala

@@ -128,6 +128,7 @@ class AkkaSpecSpec extends WordSpec with MustMatchers {
         implicit val davyJones = otherSystem.actorOf(Props(new Actor {
           def receive = {
             case m: DeadLetter ⇒ locker :+= m
+            case "Die!"        ⇒ sender ! "finally gone"; self.stop()
           }
         }), "davyJones")
 
@@ -148,6 +149,7 @@ class AkkaSpecSpec extends WordSpec with MustMatchers {
         system.registerOnTermination(latch.countDown())
         system.stop()
         latch.await(2 seconds)
+        (davyJones ? "Die!").get must be === "finally gone"
 
         // this will typically also contain log messages which were sent after the logger shutdown
         locker must contain(DeadLetter(42, davyJones, probe.ref))