Fix LFS CI fetch

What does this MR do?

Fixes LFS fetching for CI objects that were kind of broken.

Why was this MR needed?

This also refactors LFS tests to be a blackbox tests instead of whitebox tests. Actually testing GrackAuth instead of LFSRouter.

Related isssues

Resolves: https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1199

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together