Fix failing tests by running assertion in next run-loop

Some tests are failed in 2bcc7677.
This cause is that the timing for executing callback by `window.async`
got later than as expectation.
- https://github.com/emberjs/data/commit/2bcc767755a03e985a7e26ec2eee41cc1f8dc177#L3R33

So I use `Ember.run.next` to adjust timing for assertion.