Improve testing

[SlowMo24]

We currently use integration testing nearly everywhere but at the same time not properly.

• swap integration test (using VCR) for unit-testing (testing only the method of concern) where possible.
• fix cassette matching for VCR. Currently nearly all cassettes are matched/found based on the full request. Yet, many aspects of the request like header (containing the ohsome-py version) do not matter for the test itself. E.g. if the functionality of an endpoint is tested and that test is independent of the header, the cassette matching should only be done on the endpoint, not the header (https://cran.r-project.org/web/packages/vcr/vignettes/request_matching.html).

[SlowMo24]

• check all tests for consistency, duplications and coherence
• make sure all possible exceptions are also tested (white box testing)