Use PostgreSQL backend for django database in tests

Django database for swh-web has been recently migrated from sqlite3 backend to PostgreSQL one (swh/infra/sysadm-environment#2945 (closed)) in production.

Nevertheless, swh-web tests still use the sqlite3 backend.

We should also use the PostgreSQL backend in tests to align with production and catch possible issues prior deployment.

The pytest-django fixtures db and more advanced ones should help setting the PostgreSQL database for tests.

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Migrated from T3601 (view on Phabricator)