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?
I haven't used RSpec in a minute, and this is definitely just a style thing, but is there a reason to use `RSpec.describe` and not just `describe`?