I think you need both. You need randomised tests that will get you to cornercases. You also need fixed tests, i.e., every random test that ever failed, should be made into a fixed test. Then you are certain to avoid regressions, while still exploring the complete spectrum on every test run.

Which test cases? The hidden ones? They include a simple solution for random checks. The example test cases? In this case you should probably add a note how they reveal an answer.

You import sortBy, yet you do not use it, instead opting for the more expensive reverse after the sort.

I think this will walk the first k elements twice. Once to take i elements off the list and once to compute last.