keeping the "test out of this skill" button after you pass a level
I think keeping the "test out of this skill" button available after you complete the lessons would be great, you can test levels you passed a while ago and see if you are still "in shape"
Why not just review the skill? By clicking onto the trophy you can refresh the skill by doing questions from various parts of the lesson. If you do that you'll know if you're still "in shape".
I hope this helped. :)