Fix double start of test cluster

buildSrc/src/main/java/org/elasticsearch/gradle/testclusters/ElasticsearchCluster.java

@@ -301,7 +301,10 @@ private void commonNodeConfig() {
             }
             if (firstNode == null) {
                 firstNode = node;
-                firstNode.start();
+                if (node.getVersion().before("6.5.0")) {
+                    // We need to start the first node early to be able to provide unicast.hosts
+                    firstNode.start();
+                }
             }
         }
     }