Doc updates for ember-testing/lib/test.js.

**tldr;** Only documentation changes are included in this PR, but I believe that additional semi-structural changes need to be made so that the docs will display properly.

Specifically:

* We should separate the items [reopening Ember.Application](https://github.com/emberjs/ember.js/blob/3070328313b9c442d42b4ffb38790ee4683c2b41/packages/ember-testing/lib/test.js#L154-#L192) into a separate file.
* That file should define `Ember.Testing.ApplicationMixin` or something like that.
* Update [Ember.Application](https://github.com/emberjs/ember.js/blob/f56e0fcc39ff2936aa18bad163552c64be8e673a/packages/ember-application/lib/system/application.js#L162) docs to state: `@uses Ember.Testing.ApplicationMixin`

Once done the API docs will display the various methods defined in the new `ApplicationMixin` (or whatever we call it) directly under `Ember.Application` (and also under `ApplicationMixin`).

These changes seem documentation only, but I did not want to try to make them without double checking first.  I am more than happy to complete these changes (and I have already tested the `YUI` output to confirm how it will display) if you would like...