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.