indexer.tests: Use module's `get_<storage-backend>` to instantiate backend
Instead of directly import the storage backend whose name could change.
This fixes the ci build [1]
Test Plan
tox
Unsure of the result yet as another error is entangled within. Meaning the build will fail as long as the other error is not fixed.
Migrated from D2608 (view on Phabricator)