Use RSVP.Promise.resolve().then(...) to decrement AJAX counters.

addon-test-support/wait.js

@@ -1,6 +1,6 @@
 /* globals self */
 
-import { next, run } from '@ember/runloop';
+import { run } from '@ember/runloop';
 
 import { Promise as EmberPromise } from 'rsvp';
 import jQuery from 'jquery';
@@ -13,9 +13,18 @@ function incrementAjaxPendingRequests(_, xhr) {
 }
 
 function decrementAjaxPendingRequests(_, xhr) {
-  next(() => {
+  // In most Ember versions to date (current version is 2.16) RSVP promises are
+  // configured to flush in the actions queue of the Ember run loop, however it
+  // is possible that in the future this changes to use "true" micro-task
+  // queues.
+  //
+  // The entire point here, is that _whenever_ promises are resolved, this
+  // counter will decrement. In the specific case of AJAX, this means that any
+  // promises chained off of `$.ajax` will properly have their `.then` called
+  // _before_ this is decremented (and testing continues)
+  EmberPromise.resolve().then(() => {
     for (var i = 0; i < requests.length; i++) {
-     if (xhr === requests[i]) {
+      if (xhr === requests[i]) {
         requests.splice(i, 1);
       }
     }