Convert to RSpec

This is still a work in progress, but I thought I'd go ahead and put this here so folks can discuss.

Basically: I'm better at RSpec than MiniTest. I find it easier to write more expressive tests and so I feel more confident about covering edge cases in it. Also, this sounded better to me than chasing down what part of our testing infrastructure was messing things up in my efforts to get tests running across multiple versions of ActiveRecord in #47.

I don't want to merge this without at least some quorum of the other commiters agreeing that this is reasonable. So: What do folks think about RSpec?
It's slightly more concise (it looks _way_ more concise, but only because I split things up across lines), but also potentially more leaky. RSpec will make sure `let` and `let!` stuff is only in scope for the right tests, but instance variables will stick around and possibly cause order-of-running weirdness.

I have an idea that later on, we can factor out a lot of this `let!` crap into factories or somehow else reduce the number of special helper classes and stuff. Heck, it might be that we can redo this stuff so that we don't have to refer to them by name somehow. But I wanted to keep the new tests _pretty_ close to the old ones in this first step.