Extend API tests

[cmil]

This PR adds documentation for possible 404 responses, sets up a test instance that can be started in parallel to a development one (port 8081), and runs all test types except status_code_conformance. The latter still yields several undocumented 400 Bad request responses.

see #236

[afuetterer]

There are five check groups of which you use four.
You can also use a reverse approach and use "--exclude-checks" if you like.

[cmil]

There are five check groups of which you use four.
You can also use a reverse approach and use "--exclude-checks" if you like.

Thanks for the hint. The goal is to eventually use all checks. Right now there are still tests failing if we do that, which is why I put the PR in draft mode. I'll try to improve this as soon as time allows.

[afuetterer]

Very nice.