Harden RemoteFailureSpec, #31250 (#31709)

* Reduced to 3 remote ActorSystems, in case it was too heavy to
  run 6 in CI
* Adding a first message identification check (at least easier to diagnose)

akka-remote/src/test/scala/akka/remote/artery/RemoteFailureSpec.scala

@@ -6,6 +6,8 @@ package akka.remote.artery
 
 import scala.concurrent.duration._
 
+import akka.actor.ActorIdentity
+import akka.actor.Identify
 import akka.remote.EndpointDisassociatedException
 import akka.serialization.jackson.CborSerializable
 import akka.testkit.{ EventFilter, ImplicitSender, TestActors, TestEvent }
@@ -24,15 +26,19 @@ class RemoteFailureSpec extends ArteryMultiNodeSpec with ImplicitSender {
   "Remoting" should {
 
     "not be exhausted by sending to broken connections" in {
-      val remoteSystems = Vector.fill(5)(newRemoteSystem())
+      val remoteSystems = Vector.fill(3)(newRemoteSystem())
 
       remoteSystems.foreach { sys =>
         sys.eventStream.publish(TestEvent
           .Mute(EventFilter[EndpointDisassociatedException](), EventFilter.warning(pattern = "received dead letter.*")))
         sys.actorOf(TestActors.echoActorProps, name = "echo")
       }
       val remoteSelections = remoteSystems.map { sys =>
-        system.actorSelection(rootActorPath(sys) / "user" / "echo")
+        val sel = system.actorSelection(rootActorPath(sys) / "user" / "echo")
+        // verify that it's are there
+        sel ! Identify(sel.toSerializationFormat)
+        expectMsgType[ActorIdentity].ref.isDefined should ===(true)
+        sel
       }
 
       system.actorOf(TestActors.echoActorProps, name = "echo")