Add tests

[Jojodicus]

For each change and feature added, things have to be tested manually.

This was fine when the feature-set was small and stability wasn't really cared about, but now as CalUnite mostly hit a production-ready state and is used by multiple people that rely on it, proper validation is a priority.

[Jojodicus]

Will probably do in a pipeline as well.

Since I like https://goreportcard.com/report/github.com/Jojodicus/calunite, might be some more things to check automatically