Squishy minitest5

I've been working with @tenderlove and @steveklabnik on this so it already has eyeballs on it, but the more the merrier.

There is more that I can do **after** this to improve stability of the tests as a whole. There are hacks in AS::TestCase to deal with test order dependencies that should go, but that's a problem with the tests themselves that I can deal with later.