[Dev] create tests for the plugin.

[Balvald]

I as the developer would like to have both unit tests and some other tests that simulate certain common and uncommon user behaviours that the plugin would need to face so that when I work on every next iteration of the plugin I don't have to do some tests manually and end up forgetting some obvious or important case and introduce new bugs into the project.

This one is free form no milestone attached to it. Though I've hinted I'll do this after v0.2.5 though probably will do atleast some of it during this.

• [x] #41
• [x] #42
• [x] #43

Best would be also to introduce tests to prevent bugs from resurfacing so a list of bugs that need their tests on the other repo once its created:

• [ ] #12
• [ ] #32
• [ ] #37
• [ ] #35