Testing fixture to test the remote api instance exists but only in the swh-storage modules (it started there).

The remote api access pattern is shared amongst many modules (swh-indexer, swh-vault, swh-objstorage, swh-storage, swh-scheduler, etc...).
The same goes for its testing fixture.

But some modules duplicate it (e.g objstorage), some depends on swh-storage directly (e.g swh-indexer, swh-archiver)...
Moving it out of swh-storage will clarify the dependencies and reflects what is currently being done for the testing db fixture.