test: suggested updates for testing if typecheck middleware is installed

[dfreeman]

@mrloop Thanks for your patience on typed-ember#1148! We'd love to get that change in before our first 4.0 release candidate

I'm opening this PR to suggest a slightly different approach to testing whether the typecheck middleware is installed or not. You seemed to be having to jump through a lot of hoops to work around the fact that ember test in the skeleton app would never actually terminate, so this PR fixes the skeleton app's fixtures so that the tests will actually run to completion.

In addition to that change, I also updated the addon to emit debug messages indicating whether the typecheck middleware is being included or not so we have something easier to assert against.

I think those two changes together avoid a lot of the gymnastics we were forcing on you to test your change, so hopefully this is an improvement 🙂

[mrloop]

@dfreeman thanks! Especially like adding the extra debug messages, makes things explicit and simpler.