Handle async behaviour outside of tests

[mydea]

This fixes ember-engines/ember-engines#341. Not 100% sure if that might have any negative side effects, but it fixed it for me.

[trentmwillis]

One thing I worry about with this is that if you have async behavior before starting your tests, then this approach does not prevent tests from starting to run. I think that this is actually a good thing (less magic), but it is also different from previous behavior.

@rwjblue would appreciate your thoughts here. Note: I think this ties in with the ideas presented in ember-cli/ember-cli-qunit#171, as you could then have control over doing async things and determining when tests are actually loaded.

[trentmwillis]

Sorry for letting this sit so long. After reviewing again, I think my concerns noted above are not really valid.

@mydea would you mind rebasing this? If CI is still green, then I'll go ahead and merge. Thanks!

[mydea]

Done, I rebased on master, seems to still be fine!

[trentmwillis]

Thanks!