Call objstorage.content_get() with a HashDict instead of single hash
Hash dicts are now prefered by swh-objstorage, in order to support individual hash collisions.
This commit includes some minor fixture refactoring, to allow tests to mock methods of the backend class even in proxy tests.
Test Plan
Depends on swh-objstorage!120 (closed)
Migrated from D8127 (view on Phabricator)