Refactor cluster startup join in tests and fix barrier race

* Refactored common code to MultiNodeClusterSpec.awaitClusterUp
* Fixed some race conditions of barriers

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

@@ -30,15 +30,39 @@ object MultiNodeClusterSpec {
 trait MultiNodeClusterSpec { self: MultiNodeSpec ⇒
 
   /**
-   * Create a cluster node using 'Cluster(system)'.
+   * Get or create a cluster node using 'Cluster(system)' extension.
    */
   def cluster: Cluster = Cluster(system)
 
   /**
-   * Use this method instead of 'cluster.self'.
+   * Use this method instead of 'cluster.self'
+   * for the initial startup of the cluster node.
    */
   def startClusterNode(): Unit = cluster.self
 
+  def startCluster(roles: RoleName*): Unit = {
+    awaitStartCluster(false, roles.toSeq)
+  }
+
+  def awaitClusterUp(roles: RoleName*): Unit = {
+    awaitStartCluster(true, roles.toSeq)
+  }
+
+  private def awaitStartCluster(upConvergence: Boolean = true, roles: Seq[RoleName]): Unit = {
+    runOn(roles.head) {
+      // make sure that the node-to-join is started before other join
+      startClusterNode()
+    }
+    testConductor.enter(roles.head.name + "-started")
+    if (roles.tail.contains(myself)) {
+      cluster.join(node(roles.head).address)
+    }
+    if (upConvergence && roles.contains(myself)) {
+      awaitUpConvergence(numberOfMembers = roles.length)
+    }
+    testConductor.enter(roles.map(_.name).mkString("-") + "-joined")
+  }
+
   /**
    * Assert that the member addresses match the expected addresses in the
    * sort order used by the cluster.