Use the in-memory storage instead of mocks to test the webapp

Currently, the unit tests of swh-web heavily rely on mocking the methods of the swh.storage.Storage class. While the approach works, there is a couple of drawbacks with it:

• tests implementation are hard to read and maintain for a newcomer
• tests data are hardcoded and can end up not synchronized with the latest swh-storage schema
• maintaining tests full of mocks is painful and too much time consuming

These tests must be rewritten using the new in-memory storage.

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Migrated from T1271 (view on Phabricator)