Fix regression in Cluster Singleton, #21236

* When the test fails the node is removed from the membership
  twice, which triggers two OldestChanged cycles, but in
  the 2.4.9 change https://github.com/akka/akka/pull/21152/files#diff-f0ae95c926a050aecf45dba3e08d1c77L669
  the singleton manager always goes to End (stop) when it has been Oldest
* This fix restores the previous behavior for this scenario

akka-cluster-tools/src/main/scala/akka/cluster/singleton/ClusterSingletonManager.scala

@@ -608,6 +608,9 @@ class ClusterSingletonManager(
           // already oldest
           stay
         case Some(a) if !selfExited && removed.contains(a) ⇒
+          // The member removal was not completed and the old removed node is considered
+          // oldest again. Safest is to terminate the singleton instance and goto Younger.
+          // This node will become oldest again when the other is removed again.
           gotoHandingOver(singleton, singletonTerminated, None)
         case Some(a) ⇒
           // send TakeOver request in case the new oldest doesn't know previous oldest
@@ -683,7 +686,9 @@ class ClusterSingletonManager(
     if (removed.contains(cluster.selfUniqueAddress)) {
       logInfo("Self removed, stopping ClusterSingletonManager")
       stop()
-    } else
+    } else if (handOverTo.isEmpty)
+      goto(Younger) using YoungerData(None)
+    else
       goto(End) using EndData
   }