Registered test waiter for ember-raf-scheduler

no issue
- recommended as per `vertical-collection` docs: https://github.com/html-next/vertical-collection#testing
- ensures Ember's `wait` helper is aware of any raf-scheduler loops

ghost/admin/tests/test-helper.js

@@ -1,9 +1,12 @@
 import Application from '../app';
 import config from '../config/environment';
+import registerWaiter from 'ember-raf-scheduler/test-support/register-waiter';
 import {setApplication} from '@ember/test-helpers';
 
 setApplication(Application.create(config.APP));
 
+registerWaiter();
+
 mocha.setup({
     timeout: 15000,
     slow: 500