Update tests to follow RemoteObjStorage api fixes

Summary:
As the api of RemoteObjStorage have been fixed (See D83) and is now
aligned to the base ObjStorage one, places where the remote API were
used directly must be updated.

Related D83

Reviewers: Reviewers!, olasd

Maniphest Tasks: T513

Differential Revision: https://forge.softwareheritage.org/D84