Show diff between 2 CI runs on Slack reports
# What does this PR do?

It has now become more difficult to identify the **new** CI failures on Slack reports, as the number of failures is in the range of `[100, 200]` + the number of rows in the reported table is kept at around `40`.

This PR adds a **diff** of the **(model) failure tables** reported by the latest run against by the last previous run.

### The effect

<img width="720" alt="Screenshot 2023-04-17 060007" src="https://user-images.githubusercontent.com/2521628/232374887-66f9259b-9878-4ade-b337-553d3d34dc71.png">

_The documentation is not available anymore as the PR was closed or merged._