create a "mock" backend and use it to simmulate a full pipeline run

[6543]

similar to an full integration test but as "normal" unit test, i want tests that cover the whole pipeline creation, execution and report.

this should be posible as soon as we have a "mocked" backend where we can simullate a running e.g. docker agent.

this should also catch things like #3494 ...

TODOs:

• [ ] create a mock-backend
• [ ] write framework around that idea ...