Add missing security specs for raw snippet access

What does this MR do?

It extends the project snippets access security specs to cover raw snippet paths as well.

When I was researching snippets for !7256 (merged), I noticed that specs existed for the HTML show view of project snippets but not the raw view. Seeing as this is a spec that is checking for access regressions on places where sensitive information might be kept, I thought it would be a good idea to cover the raw snippets access too.

To balance out the karma of adding in extra tests I also changed the tests to all use an empty_project spec.

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

With the aim of making the specs easier to read, I restructured some of them to use context blocks for each type of snippet.

I've used the same access rights defined for the show snippet paths for the raw snippet access.

Why was this MR needed?

To catch security regressions on raw snippet access for projects.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• [-] CHANGELOG entry added
• [-] 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?

Added 114 commits:

• 242d6d22...50f3fd49 - 113 commits from branch gitlab-org:master
• 80a2e3a9 - Add missing security specs for raw snippet access

Compare with previous version

To balance out the karma of adding in extra tests I also changed the tests to all use an empty_project spec.

Thanks, @the-undefined! I'll take a look next week.

Added ~164274 backstage labels

Reassigned to @smcgivern

Status changed to merged

Mentioned in commit 7ce03197

changed milestone to %8.14