skip_gitaly_mock disables Gitaly in specs

What does this MR do?

Makes :skip_gitaly_mock disable Gitaly instead of just returning whatever it "normaly would". This is required since we now have "opt-out" migrations that needs to be disabled inside some tests.

See e.g. https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/40314908

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered