Add ability to skip key result tests or fix afforestation model issue

[denised]

Afforestation model fails key result tests because the python key result code explicitly skips certain "nonsense" situations, such as having a replacement lifetime value of 0, which Afforestation has. It might be possible to make the key results match the excel behavior, but its not certain that is worthwhile. Or, we could allow marking specific key results as accepted failures in the same way we do deep results.