Add post-job validation unit tests
We currently have unit tests to cover the yaml validation etc... that just study the input of a job but some validation methods (similar to checks) now rely on studying the output of a job rather than just its input. It should be enough to set up a job specifically for these tests and to store their relevant output in the repository (if they're not too large) for the validation to then study.
Work towards task https://gitlab.cern.ch/lhcb-dpa/project/-/issues/93.
@erodrigu this should help improve our overall coverage.