tests: Fix unchainable `helper.waitFor().fail()`

The `helper.waitFor()` function returns a jQuery Deferred object.
Deferred objects are supposed to have a `.fail()` method that is
chainable (it should return `this`). Before this change,
`helper.waitFor()` monkey-patched the `.fail()` method with a function
that returned `undefined`. Now the monkey-patched `.fail()` returns
the Deferred object.

Also modernize the code a bit.

tests/frontend/helper.js

@@ -164,12 +164,12 @@ var helper = {};
 
     var deferred = $.Deferred();
 
-    var _fail = deferred.fail;
+    const _fail = deferred.fail.bind(deferred);
     var listenForFail = false;
-    deferred.fail = function(){
+    deferred.fail = (...args) => {
       listenForFail = true;
-      _fail.apply(this, arguments);
-    }
+      return _fail(...args);
+    };
 
     var intervalCheck = setInterval(function(){
       var passed = false;