Implement CI tests against dockerized SODAR

[holtgrewe]

Is your feature request related to a problem? Please describe.
At the moment we use requests_mock for stubbing out REST calls to SODAR. Thus the tests depend on the test data being correct more than correct behaviour when woring with REST API.

Describe the solution you'd like
Implement tests that work against a dockerized version of SODAR. We either need a dedicated "client CI" mode in SODAR or (maybe preferred) reset the database before each test by performing a fresh setup.

Describe alternatives you've considered
We could keep things as they are or implement a fake SODAR server. Both have their problems.

Additional context
Depends on creation of https://github.com/bihealth/sodar-docker-compose