Not an issue

Do you know which language this was in?

Hi, I'm the original author of this Kata (Ruby version). Author of the haskell version didn't understand this Kata properly, and indeed added a DFS version instead of BFS.

I have learned haskell to fix this. I added a BFS solution and a correct buildTree (thanks to @nickie for providing one). Please take a look again.

Thanks,
Karol

Same issue

Also validation should run tests (the same way as when user presses "Run tests") with complete solution.

Actually, some katas expect the user to find a solution, not to create a program. There are not viable example test cases in that scenario. If the kata validation used Run tests, it would fail for all those katas that don't have example test cases.