Add more tests with async loaded routes.

Creates a separate set of tests that leverage async loaded routes
(roughly akin to what ember-engines does) to ensure model loading and
loading states work correctly in that scenario.

Currently, this emits the following error:

```
TypeError: Cannot read property '_stashNames' of undefined
    at stashParamNames (http://localhost:7021/tests/ember.js:23926:13)
    at Class.setup (http://localhost:7021/tests/ember.js:20365:37)
    at _routeEnteredOrUpdated (http://localhost:7021/tests/ember.js:62215:17)
    at PrivateRouter.routeEnteredOrUpdated (http://localhost:7021/tests/ember.js:62230:9)
    at PrivateRouter.setupContexts (http://localhost:7021/tests/ember.js:62159:16)
    at PrivateRouter.getTransitionByIntent (http://localhost:7021/tests/ember.js:61969:14)
    at PrivateRouter.transitionByIntent (http://localhost:7021/tests/ember.js:61905:21)
    at PrivateRouter.doTransition (http://localhost:7021/tests/ember.js:62040:19)
    at PrivateRouter.intermediateTransitionTo (http://localhost:7021/tests/ember.js:62522:19)
    at Class.intermediateTransitionTo (http://localhost:7021/tests/ember.js:22389:28)
```