Async: Make tests Promise-aware

Fixes #632.
Ref #534.

~~I'm **not** loving how un-DRY the `Test#run` function is... but that's not _new_, only exacerbated by this PR.~~

**Updated:** This makes the `setup`, `teardown`, and `test` methods all aware that their return value may be a Promise in need of asynchronous resolution (`resolve`-ing).

Criticism or other suggestions welcomed, as usual.

cc: @domenic @stefanpenner
Possibly, though that would be inconsistent with the other async scenarios.