[META] Improve the testing guides

[acorncom]

We'd like to see the discussion of testing in guides fleshed out in more detail (to encourage folks to test a variety of places). Please comment here as you see things that need to be done in more detail and we'll add them to the checklist below:

• [ ] add helper unit test example (example here: http://stackoverflow.com/questions/38985836/unit-test-for-helpers-ember-cli)
• [ ] need to add documentation on unit test needs as well
• [ ] when/why use use Ember.run (link to run loop explanation)
• [ ] troubleshooting test failures
• [ ] unit testing a mixin, serializer, adapter
• [ ] writing test helpers for unit, integration tests
• [ ] ??

[skitterm]

As a newcomer to Ember, I'd like clarification in the docs on where I can use async test helpers (can I only use them in acceptance tests since that's where they're talked about?) and the best way to mimic clicks/keypresses/etc in integration tests (thought this$('#my-selector').click() was the way to go but according to #1815 it's not?)

[skitterm]

And in general, how to test async stuff in Ember would be helpful. I got confused over when to use andThen vs wait and when to/not to use the Run Loop.