[Epic] Test Suite for Engine Castor

Parent: [EPIC] Basic test coverage for all services (aura#175)

Currently the repository does not provide test cases. In other repositories we are aiming for approx. 80%+ coverage.

Sub Tasks

• Implement unit tests where meaningful
• Implement integration tests where meaningful (excluding other services)
• Add some test coverage extension and display the coverage in the README.md (compare other repositories).