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?

changed title from Use empty_project for resources that are independent of the repo to Improve discussion_comments_spec perf

added 1 commit

• 7e525015 - Collapse similar sibling scenarios

Compare with previous version

added 1 commit

• 15914001 - Separate examples for each other

Compare with previous version

added 1 commit

• 15914001 - Separate examples for each other

Compare with previous version

assigned to @DouweM

This should be picked into 9-1-stable to cut a few minutes of the build time.

added ~149423 label

changed milestone to %9.1

merged

mentioned in commit 50715a92

Picked into 9-1-stable, will go into 9.1.0-rc2

mentioned in commit f8df3559