Make it easier to set up predictable median harvest time in specs

[CloCkWeRX]

For https://github.com/Growstuff/growstuff/pull/3623/files#diff-bb9fcf8d7443df378cf564c02a2ac5b12bf5b036a595fce0303a595060199656L17

I just went right to the DB and stuck in the medians, because I was caught off guard by the predictable_planting factory not actually being predictable for harvests.

https://github.com/Growstuff/growstuff/blob/dev/spec/features/plantings/prediction_spec.rb#L6 shows the full amount of data you have to set up, and it's ... a lot.

Either:

Make the factory/traits for a predictable planting also set up harvests or fake it similar to how I did.