Test files are picked up as ember core modules

Currently tests are being picked up by the [isEmberCoreModule](https://github.com/ember-cli/eslint-plugin-ember/blob/master/lib/utils/ember.js#L115) check, possibly triggering rules, depending on the code in the test.

Given some test for a controller like this, it will be picked up as an ember controller because of the path and trigger the [alias-model-in-controller](https://github.com/ember-cli/eslint-plugin-ember/blob/master/lib/rules/alias-model-in-controller.js) rule.
```
import { moduleFor, test } from 'ember-qunit';

const somethingThatNeedsToUseExtend = Ember.Object.extend({...});

moduleFor('controller:my-controller', 'Unit | Controller | my controller', {
...
```

I wll submit a PR, but excluding files with `tests` in the path from the `isEmberCoreModule` check would solve this.