conftest: use pytest-postgresql for django related tests
Replace the (non-working) django init hook by overloaded django_db_setup and make this django_db_setup use pytest-postgresql so we do not depend on external test environment (running postgresql server, tox or hand managed pifpaf).
Also make tests run with a proper scheduler so the scheduler interaction code is executed.
Note that this does not test for correctness in these interactions yet.
also move tests/init.py content in tests/conftest.py and adapt test code accordingly.
This also ensures retries_left is set otherwise tests may fail when using the local sheduler.
Migrated from D2474 (view on Phabricator)