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.

Criticism or other suggestions welcomed, as usual.

cc: @domenic @stefanpenner
Why do you say that? This particular test is all about testing `setup`'s Promise-awareness.  The unit tests for testing `test`'s Promise-awareness are below.