WIP: Diff comments feature test

Closed Luke "Jared" Bennett requested to merge diff-comments-test into master

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?

/cc @fatihacet @DouweM

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