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
in theory people can implement a custom "then" that has side-effects when invoked. This can be malicious, in-error, or some other funkyness.