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
Let's drop this one, since there's an assertion here anyway. This kind of marker ("I was here!") is only useful when there's nothing else.