Write functional unit tests

[rikkolovescats]

right now plugin manager only tests whether the json is correctly formatted and that it doesn't break for everyone using the plugin manager; in addition to this, it might also be nice to have functional tests which can check whether plugins are actually visible in the in-game plugin manager, are installable, etc.

we'll have to integrate this kind of tests with bs_headless somehow so they can execute headlessly on github actions, etc.