Check that the forecast tests introduced in #2318 catch bug fixed in #2312

https://github.com/input-output-hk/ouroboros-network/pull/2312 fixed a bug in the forecasting infrastructure and introduced `TranslateForecast`. During my work on the forecasting tests, I realized that that`TranslateForecast` wasn't quite perfect yet, and I modifed it slightly in https://github.com/input-output-hk/ouroboros-network/pull/2318. However, it would be good to verify that the new tests can actually catch this particular bug, and if not, try to strengthen them.

I don't think this is terribly high priority though; the new forecasting tests are quite thorough.