Enhance e2e tests with mocked feeds

[jcpitre]

Describe the problem

In some issues (e.g. #1885) we have a problem that does not manifest itself in any of the datasets in the database, but could happen if the right corner case is present.

Proposed solution

We should consider having a list of mock feeds with contents covering these corner cases.
We could validate these feeds with each PR, and make sure the errors either manifest themselves if the problem is not solved, or not manifest themselves if the problem is solved.

Probably the easiest way to do that would be do a comparison similar to the Acceptance test report.
Except we would need to point out any differences, instead of just displaying a percentage of change like the acceptance tests run on the database.

Alternatives you've considered

No response

Additional context

No response