Unit tests for GrowBikeNet

[Manuel-Knepper]

Have functioning unit tests via PyTest

[anastassiavybornova]

@Manuel-Knepper i think it would be great, as we implement this, to also add bulletpoint style instructions to the contributing guide on how to run the tests (documenting our own workflow)

[anastassiavybornova]

goal for W7: have a functioning test environment