Tests: Add a lot more test coverage.

[ryan-roemer]

[parkerziegler]

@ryan-roemer @ElreyB is taking awesome initiative on this ticket. Is there anything in particular you'd like him to focus on with tests (either between the plugin or the UI itself)? Any guidance here would be great 🎉

[ryan-roemer]

@ElreyB -- ping me next week and hopefully we can chat! (I'll be back next week)

[parkerziegler]

@ryan-roemer Latest coverage summary after merging @ElreyB's PR:

============ Coverage summary ================
Statements   : 37.8% ( 144/381 )
Branches     : 43.03% ( 71/165 )
Functions    : 18.87% ( 20/106 )
Lines        : 38.48% ( 142/369 )
===========================================

So still some way to go, but it's getting there. Going to leave this issue open as a tracking place for test PRs.