Switch testing from sortOn to sortWith

[treeowl]

Data.List.sortOn uses a decorate-sort-undecorate algorithm. That's great when the passed function is expensive. But when it's very cheap, it just wastes time and space. In our tests, we have a bunch of sortOn Down. Since Down is just a coercion, this is lousy. We should instead use GHC.Exts.sortWith, which applies the passed function each time.

[konsumlamm]

The tests already run pretty fast (~0.2s) and using sortWith makes no difference for me.

[treeowl]

I'd like to do it anyway. Do you mind for some reason?

[konsumlamm]

No, feel free to make a PR.