Idea: Make a unit test directory similar to the rules directory

[tnelling]

0.25-block-test-failures.py is a great idea, but as we add more tests we'll probably end up with unnecessary merge conflicts between unrelated tests. As with the rules, we could instead have a tests directory that we can just add new tests to. This could be a directory of directories with a folder per rule and a file per test definition, or we could just have one file per rule and all that rule's tests specified together.