Implement Backend Tests

[tinxx]

The Backend server currently has zero test coverage. This should be changed.

Testing Spring Boot applications is not always straight forward as there are several dependencies.
However, the Backend uses an in-memory SQLite instance by default so it should not be necessary to change that for testing.

It has to be decided if tests should cover actual container actions or if those should be mocked.

While implementing tests, consider updating or extending code documentation (JavaDoc, READMEs) if something is not quite right, ambiguous, or missing.

[koschke]

I'd love to have the coverage data covered by JaCoCo because we can import JaCoCo test reports and visualize the data in our code cities.