WIP workflows: test teardown fix

* Fixes ordering of objects being deleted in the test teardown.

Reported-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
`BibWorkflowObject ` as well as `BibWorkflowEngineLog` and `BibWorkflowObjectLog` should be configured according to http://docs.sqlalchemy.org/en/rel_0_9/orm/tutorial.html#configuring-delete-delete-orphan-cascade