Add unit and integration tests for all endpoints (help welcome)

[adewes]

We should add unit and integration tests to all API endpoints, or at least all endpoints that touch sensitive user/provider data. There are several examples already but they do not cover all important cases. To finish the testing we need to define additional fixtures and finish the client functionality.

I self-assigned this issue as I will use this as an opportunity to refactor some code but help is welcome!