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
We once use `test = this;` elsewhere in this file, I suggest using the same here. That's consistent with `assert = this;` in assert.js.