Stop actor system from shutting down on Cluster.leave (#23872)

This then sets a race bewtween the rest of the test running as once the
ActorSystem shuts down test test coordinator won't for for barriers etc.

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

@@ -3,12 +3,9 @@
  */
 package akka.cluster
 
-import scala.collection.immutable.SortedSet
-import com.typesafe.config.ConfigFactory
 import akka.remote.testkit.MultiNodeConfig
 import akka.remote.testkit.MultiNodeSpec
 import akka.testkit._
-import scala.concurrent.duration._
 import akka.actor.Props
 import akka.actor.Actor
 import akka.cluster.MemberStatus._
@@ -62,7 +59,6 @@ abstract class NodeLeavingAndExitingSpec
 
         // Verify that 'second' node is set to EXITING
         exitingLatch.await
-
       }
 
       // node that is leaving