Add downstream test for monorepos

What does this MR do?

Add downstream test to lock in support for monorepos.

What are the relevant issue numbers?

gitlab-org/gitlab#281593 (closed)

Does this MR meet the acceptance criteria?

• [-] Changelog entry added
• [-] Documentation created/updated for GitLab EE, if necessary
• [-] Documentation created/updated for this project, if necessary
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Job definition updated, if necessary
  • [-] Auto-DevOps template (also in CE)
  • [-] Job definition example
  • [-] CI Templates (to be removed)
  • [-] Vendored CI Templates (also in CE)
• [-] Conforms to the code review guidelines
• [-] Conforms to the Go guidelines
• Security reports checked/validated by reviewer