WIP: Diff comments feature test

THIS MR IS BRANCHED FROM !5262 (merged) WHICH IS CURRENTLY UNMERGED!

What does this MR do?

Adds tests for the diff comments feature.

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

😕

Why was this MR needed?

Coverage plus regression-prone feature.

What are the relevant issue numbers?

Related to #19824 (closed). Related to !5262 (merged).

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 style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

/cc @fatihacet @DouweM