tests: Use production backends within the indexer tests
This detected some paper cuts within the db.py module (through some cli tests no longer using the memory storage).
The main goal is to decrease friction when actually deploying indexer related services (backend, indexers, ...).
The pg backend tests should still be reasonably fast as it's using the swh.core.db.pytest_plugin (which truncate tables in between tests).
Indeed, a rapid look in the jenkins diff ui [1] shows that the overall execution time is reasonably as fast as before.
One of the next steps would be to improve the current journal client tests in the indexer [2]
Related to T2821
Test Plan
tox
Migrated from D4638 (view on Phabricator)