Tweak ember-qunit config

Following
[migration](https://github.com/emberjs/ember-qunit/blob/master/docs/migration.md) steps `4.x` -> `5.x`

Author: vladucu

tests/index.html

@@ -21,6 +21,13 @@
     {{content-for "body"}}
     {{content-for "test-body"}}
 
+    <div id="qunit"></div>
+    <div id="qunit-fixture">
+      <div id="ember-testing-container">
+        <div id="ember-testing"></div>
+      </div>
+    </div>
+
     <script src="/testem.js" integrity=""></script>
     <script src="{{rootURL}}assets/vendor.js"></script>
     <script src="{{rootURL}}assets/test-support.js"></script>

tests/test-helper.js

@@ -2,6 +2,10 @@ import Application from 'dummy/app';
 import config from 'dummy/config/environment';
 import { setApplication } from '@ember/test-helpers';
 import { start } from 'ember-qunit';
+import * as QUnit from 'qunit';
+import { setup } from 'qunit-dom';
+
+setup(QUnit.assert);
 
 setApplication(Application.create(config.APP));