indexer: Improve tests

Some are using only in-memory implementations. Which are quite fine to go fast. They don't necessarily catch real production issues.

Some other tests are using mock tests (journal ones). From experience, the mocks are brittle when it comes to catching eventual breaking evolution changes over time.

In general, reworking those tests to use actual data services would be preferrable

Related to #2814 (closed)

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Migrated from T2821 (view on Phabricator)