Improve Test Harness and Add More Tests

The tests have been useful for catching regressions when things are refactored. Having tests for all features of all core plugins would be great.

It also might be nice to have a separate fast test command/argument that only does the small file tests (the gigantic file tests take a while to run and will usually only catch problems in the BitArray caching system.)

Start by locating the existing test harness, core plugin tests, and the command or argument used to run tests. Review how small and gigantic file tests are selected, then define completion as coverage for all core plugin features plus a fast mode that skips the gigantic file tests while retaining the relevant BitArray caching checks.

Written by the indexing model from the issue text.