Convert tests to TestUnit

I think we made a mistake using MiniTest in the tests. It meant that there were suddenly multiple testing syntaxes available with new and (potentially) unknown assertions. Also, the spec style syntax is great but require careful coordination. For instance, do you include "should" in names? How far do you nest describe blocks, if at all? Also, for that spec style RSpec 3 is better than MiniTest (no Object assertions, faster).

These commits make the stack:

* Simpler (default stack, less "new" things) so it's easier for people to get up and running.
* Run 4-5x faster.
* Autotest can be run without being included in the Gemfile (which means smaller slugs and deploy times)