Remove global test helpers.

Remove in favor of using https://github.com/rpflorence/ember-qunit/pull/46 and https://github.com/abuiles/ember-test-helpers, basically I'll maintain `ember-test-helpers` for helpers which are not attached to any testing framework and can be used across different projects.

```js
import { getAssertionMessage, equal, strictEqual } from 'qunit';
import  exists from 'ember-test-helpers';
```