Create "backend integration test" framework

[6543]

and address

• create a "mock" backend and use it to simmulate a full pipeline run #3503 -> Add dummy backend #3820
• Add api for forges #3733 (comment)
• ...

Tests to run:

• Run workflow with steps that should only run if the step before [failed,succeded] (could help with questions like: Remove some ci environment variables #3846 (comment))
• Services and check if it is started before the steps who need it ...
• Make sure the Queue respect labels and dependencies correctly
• ...

[6543]

atm we only have unit tests. even the DB migration tests are written similar to unit tests (through you could can call it integration tests when non sqlite DBMS are tested ...)

but we need something witch spins up a full woodpecker instance and run tests against it like -> trigger a pipeline. via "fake" webhook. and see the result. create a pipelien witch inentional should be marked as failed etc ...

[6543]

the pipeline backend part -> #3820