Add object storage related tests for `gitlab:cleanup:project_uploads` task

What does this MR do?

I realized I didn't test behavior when object storage for uploads is enabled. This corrects that mistake, and defines the behavior (no change).

The second commit only dries up the tests because it's getting big.

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conforms to the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them