Setup Busted test framework and start porting tests

[alerque]

Here is a start at work for #356.

Porting all the tests will take some time. It isn't always very clear what is being tested or why, so setting up the right test in a new syntax sometimes takes a dig through commit and issue history. Hopefully the test descriptions in Busted will do a better job of documenting the 'why' of tests for future developers.

I suggest we port a small handful of tests to make sure the framework is working as we want, then merge this. That will leave some tests in the old system and some in the new which is awkward, but it will mean all new tests can be written in Busted. I'm already working on new tests for the class system (see #353) and I don't really want to write them in the old framework. It doesn't seem to be too awkward to have them side by side. Eventually when the old tests directory is cleared out in favor of new tests we can drop the test run command and that will be it.

• Must be merged after Tweak rockspec to normalize fields, callout spec version, etc. #357 on which it is based since there are rockspec changes.
• Port enough tests that we set a precedent we like for nested test arrangements.

[alerque]

I'm content with the general direction this is headed and organizing tests by module, then breaking some of then down by function for testing.

To anybody else reviewing this: it's probably easiest to review commit my commit rather than looking at the PR diff. GitHub makes a mess of the overall picture, but each commit should port tests for one module or chunk thereof. Seeing the removed test(s) and added test(s) matched up makes a lot more sense.

I won't have much attention for this for a few days, but I think getting this started as the direction in master and letting new tests and test migrations come in piecemeal is okay.