Initial tests. These just try adding a few consensuses, looking
them up, and making sure that consensus diffs are generated in a
more or less reasonable-looking way. It's enough for 87% coverage,
but it leaves out a lot of functionality.
This module's job is to remember old consensus documents, to
calculate their diffs on demand, and to .
There are some incomplete points in this code; I've marked them with
"XXXX". I intend to fix them in separate commits, since I believe
doing it in separate commits will make the branch easier to review.
