Fix food fun reduction test not actually having any reduction

[RenechCDDA]

Summary

None

Purpose of change

"fun_for_food_eaten_too_often" did not actually test anything useful. It checked if a scaling penalty was applied, but the penalty was 0.

Describe the solution

Actually check to make sure the penalty exists.

Pad the test with some more sanity checking to make sure we're doing math that makes sense.

Use a copy of the test item for the test data mod to ensure this sort of shenanigans does not occur again.

Describe alternatives you've considered

I didn't even know we had a defineable "monotony_penalty", it seems nothing actually uses a manual definition.

Kind of wanted to go muck with it and make it better across the board, but my time is due for 0.H right now.

Testing

Test passes locally

Additional context

Also updated docs to explain when monotony doesn't apply, instead of just hiding it in the item factory.