Advance readiness (no need for reset).

I took some time looking into how we could make the `visit` test helper setup routing (instead of relying on `App.reset()`), and discovered that the reason we have to call `App.reset` in our `start_app` helper ([here](https://github.com/stefanpenner/ember-app-kit/blob/8d3f1c6f2fedd7d90fe8f89ed873730b3e18fb3a/tests/helpers/start_app.js#L24)) is because `setupTestHelpers` defers readiness, and without an explicit `advanceReadiness` call our application will never be able to route.  The existing call to `App.reset()` just bypassed that issue since it forcefully sets `App._readinessDeferrals` to 1 and then calls `advanceReadiness`.

This does not address the general Ember issue, but since we are creating a one-off `App` instance in this project it should be all we need for now.