Can't import test helpers from an addon.

I'm using `ember-power-select` and it works perfect inside twiddle. But now I'm trying to write an acceptance test which requires a [test helper](http://www.ember-power-select.com/docs/test-helpers) from `ember-power-select` addon.
```json
{
"version": "0.12.1",
"EmberENV": {
"FEATURES": {}
},
"options": {
"use_pods": false,
"enable-testing": true
},
"dependencies": {
"jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
"ember": "2.12.0",
"ember-template-compiler": "2.12.0",
"ember-testing": "2.12.0"
},
"addons": {
"ember-data": "2.12.1",
"ember-power-select": "1.8.1"
}
}
```