Geo: Refactor ProjectRegistryFinder spec

What does this MR do?

I refactored this spec as part of figuring out and adding a test for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5352, but it is inappropriate in that MR since that one may be picked into previous releases.

So I'm separating that refactor, to avoid excessive conflicts during any picks.

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

Why was this MR needed?

Screenshots (if relevant)

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
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?