Add tests

[lyz-code]

I have absolutely no idea of lua, I'm learning as I debug this plugin.

It may come the day that I'm able to write tests, if you're reading this and feel sorry for me and want to give your 2 cents, please make a PR with some tests.

[seflue]

Tests would make absolutly sense to increase the confidence in our changes. It would need a couple of refactorings though to make the business logic of the plugin testable.
I will think about how to make this happen. If someone has some spare time and can point to telescope extensions, which already have setup some good tests, this would be helpful.