Add (more) unit-tests

[Potherca]

This repository does not contain any unit-tests.

The provided classes (Nextcloud, NextcloudCalendar, NextcloudContacts, NextcloudProfile) contain a mix of side-effects and input/output logic.

The best method to tests this code would be explorartory: instead of writing test logic first, just call the class-under-test, observe what happens and then add asserts to document those observations.

For these tests, complexity is more an issue than scope.