Tests for diff notes (without failures )

What does this MR do?

Adds specs for diff notes.

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

We need to ensure that this doesn't break master again.

Why was this MR needed?

What are the relevant issue numbers?

Closes #20957 (closed)

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