CI: Improve testing coverage of core Halibot class

[richteer]

Right now, the core Halibot class has the worst coverage in our testing, and a lot of that is due to how it works. But that shouldn't stop us from writing useless tests anyway!

PR #113 opens up testing ._load_config() and ._write_config() , that should be an easy coverage booster (if you want to branch off that). Some of the other module loading components can probably also be tested in a vacuum, though no idea how some might react. Might be worth writing a test_config.json for some of those.