Speed up Unicorn specs by using a dummy Rack application instead of GitLab

What does this MR do?

Substitutes a minimal Rack application place of all of GitLab for our unicorn specs

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Without this, these specs are extremely slow. Booting a unicorn with the full GitLab application in CI can take 7 minutes or more.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

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

What are the relevant issue numbers?

Closes #32620 (closed)