As I read it, the point is not about using unit tests to verify correctness, but as an interactive means of learning the problem inside and out prior to finding

As I read it, the point is not about using unit tests to verify correctness, but as an interactive means of learning the problem inside and out prior to finding a complete solution. The trick is in learning what you don't know, and one can do that faster with regular feedback. There does seem to be a delicate balance between letting a student "stew" for a bit to really learn how to find and be confident in solutions, and preventing the student from heading down a wrong course. Books can never provide real feedback, I suppose, but it's a nice option to have solutions available even if students must be trusted to independently exhaust the available resources for attaining them first. Often having the solution is at least helpful as a "sanity check" when it comes to mathematics - I feel it is all too easy for beginners to make proofs on hidden and untenable assumptions and not necessarily realize it.