Avoid TestConductorTransport unless needed, see #2586

* Due to the shutdown issues the TestConductorTransport is by
  default not active, but it's easy to activate it and exception
  will be thrown if trying to use the featues that require it, i.e
  blackhole, passThrow and throttle
* Documented

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

@@ -21,6 +21,8 @@ case class UnreachableNodeRejoinsClusterMultiNodeConfig(failureDetectorPuppet: B
   val fourth = role("fourth")
 
   commonConfig(debugConfig(on = false).withFallback(MultiNodeClusterSpec.clusterConfig))
+
+  testTransport(on = true)
 }
 
 class UnreachableNodeRejoinsClusterWithFailureDetectorPuppetMultiJvmNode1 extends UnreachableNodeRejoinsClusterSpec(failureDetectorPuppet = true)