test_storage: Make swh_contents fixture generate content model objects
Note that this will be improved upon after the storage migration. We can take a look at the swh.model.tests.generate_testdata.gen_contents and make it generate BaseContent objects directly if it's possible. This was not done here so the impacts is limited to storage.
Related to #2494 (closed)
Test Plan
tox
Migrated from D3559 (view on Phabricator)