Harden EventSourcedBehaviorStashSpec, #30543 (#30644)

* remove assert on unhandled message, not important to
  what the test is verifying
* I think message 6 might not have arrived yet, and
  therefore not directed to unhandled

akka-persistence-typed-tests/src/test/scala/akka/persistence/typed/scaladsl/EventSourcedBehaviorStashSpec.scala

@@ -622,8 +622,6 @@ class EventSourcedBehaviorStashSpec
     "stop from PoisonPill after unstashing completed" in {
       val c = spawn(counter(nextPid()))
       val ackProbe = TestProbe[Ack]()
-      val unhandledProbe = createTestProbe[UnhandledMessage]()
-      system.eventStream ! EventStream.Subscribe(unhandledProbe.ref)
 
       c ! Increment("1", ackProbe.ref)
       ackProbe.expectMessage(Ack("1"))
@@ -648,8 +646,6 @@ class EventSourcedBehaviorStashSpec
 
       ackProbe.expectTerminated(c)
 
-      unhandledProbe.receiveMessage()
-
       // 6 shouldn't make it, already stopped
       ackProbe.expectNoMessage(100.millis)
     }