workflows: test teardown improvement

* Improves object deletion in the test teardown to use cascading
  deletes from workflow to objects and logs related to the Workflow and
  BibWorkflowObjects.

* Removes the unneeded test_teardown() function and updates all tests
  accordingly.

Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>