make cluster.StressSpec pass with Artery, #21458

* need to use a shared media driver to get the cpu usage
  at a reasonable level
* also changed to SleepingIdleStrategy(1 ms) when cpu-level=1
  not needed for the test to pass, but can be good to make level 1
  more extreme

akka-cluster/src/multi-jvm/scala/akka/cluster/MultiNodeClusterSpec.scala

@@ -83,12 +83,14 @@ trait MultiNodeClusterSpec extends Suite with STMultiNodeSpec with WatchedByCoro
 
   private val cachedAddresses = new ConcurrentHashMap[RoleName, Address]
 
-  override def atStartup(): Unit = {
+  override protected def atStartup(): Unit = {
     startCoroner()
     muteLog()
+    self.atStartup()
   }
 
-  override def afterTermination(): Unit = {
+  override protected def afterTermination(): Unit = {
+    self.afterTermination()
     stopCoroner()
   }