Add tests for local testing

[saileru]

We already have some integration tests which run the native-image executable.

It would be convenient to have similar tests to run and debug the current code inside the IDE during development.

This can also be an opportunity to do some refactoring, i.e. extract some functionality into separate classes, as this should improve the testability of the code.

[karottenreibe]

If you also want to do sort-of-integration-tests with a mock Teamscale server: I added one in #14 :-)