Use swh-storage PostgreSQL backend in tests

swh-web tests historically use the memory backend for swh-storage as no pytest fixture was available at the time to easily use the PostgreSQL backend.

While using memory backend is convenient, it does not allow to catch PostgreSQL related issues that could occur in production. For instance that sentry reported issue was due to an inconsistency between the memory and PostreSQL backend implementation.

Nowadays, there is a swh_storage fixture enabling to use PostgreSQL backend in tests. We should use it to align with production.

This would require to adapt the tests_data fixture and tests/data.py module implementation.

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Migrated from T3602 (view on Phabricator)