Existing run loop at the beginning of test interferes with `Ember.run.join` behavior

[ming-codes]

I'm not sure if this is intentional, but the behavior is definitely different from what ember-qunit have. Unit tests should not have a run loop started already.

I think this issue caused a test failure bug in our application test suite. A mixin's unit test had scheduled run loop actions using Ember run next. This mixin did not properly clean up those deferred actions. Later in the test suite we saw obscure run loop errors. I would expect ember-mocha to warn or error if things exist in the run loop.