fix test connection bug

public/app/plugins/datasource/opentsdb/datasource.js

@@ -167,7 +167,7 @@ function (angular, _, kbn) {
     };
 
     OpenTSDBDatasource.prototype.testDatasource = function() {
-      return this.performSuggestQuery('cpu', 'metrics').then(function () {
+      return this._performSuggestQuery('cpu', 'metrics').then(function () {
         return { status: "success", message: "Data source is working", title: "Success" };
       });
     };