Improve discussion_comments_spec perf

What does this MR do?

EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1639

https://redash.gitlab.com/dashboard/test-suite-statistics

./spec/features/discussion_comments_spec.rb 📈 😢

Steps taken:

1. Used empty_project for resources that do not depend on the repo
2. Collapsed similar sibling scenarios reducing total scenarios by ~30
3. Separate the shared example and all resource scenarios to allow for distribution across CI nodes

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
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?