Add tests

[oddstr13]

Task requirements:

• Use pytest
• requirements-dev.txt file containing the additional requirements for running tests
• This bot is currently not very testable
  Simply importing the modules is good enough for now, and even that is likely difficult for most of them.
  (application code runs on import)
• Tests should be run on pull-request and when code has been pushed to main (GitHub Actions)
  See https://github.com/jellyfin/jellyfin-kodi/blob/master/.github/workflows/test.yaml for an example

Optional, can be done later:

• Set up coverage reporting
• Lint code with black