Async: Tests are now 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
Valid point.  I was just trying to keep it close-ish to spec-compliant but it's true that, in this setup, the `return this;` serves no purpose.