BaseTestCase is complex, with a lot of arguments required for some

methods, and a lot of terminology that's required for understanding how

it all works. Most of this was undocumented, and some argument names

weren't as clear as they could be.
In preparation for some upcoming work, this change cleans up a lot of

this, adding docs that explain arguments better, reworking some function

calls to pass explicit keyword arguments to help make code more

self-documenting, and renaming some arguments to make their intentions

more clear.

Unit tests pass.