testing: Move remote api testing fixture to swh.core

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.

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Migrated from T1036 (view on Phabricator)