private void addInstancesToCluster(HelixManager helixManager, String... instances) {
    MockHelixAdmin admin =  (MockHelixAdmin)helixManager.getClusterManagmentTool();
    for (String instance : instances) {
      admin.addInstance(CLUSTER_NAME, new InstanceConfig(instance));
    }
  }
Case when no instance is down and all instances are present in ideal state or external view
@param helixManager
@param dataNodeConfigSource
@return
ambry-clustermap/src/test/java/com/github/ambry/clustermap/PropertyStoreCleanUpTaskTest.java