Add tests for the in-mem storage with origin ids disabled.
I forgot to write tests for !907 (closed)...
(note: this diff is a duplicate of D1701, but Phabricator closed the former because I committed it to another repo...)
Test Plan
They are run by tox by default (don't worry about CI run times, they only take a few seconds)
Migrated from D1717 (view on Phabricator)