Why are they being told anything outside of the test? What is that for? Isn't “can you find this bug in this file?” also a test? It sounds like there are two kinds of tests? I'm clearly confused, I realize.
They are told outside the test because if they can't find it when given hints then it's safe to assume it won't find it given no hints. It verifies to test, to an extent, much like running tests that should fail when given a set of inputs that should make it fail (you write an always failing test alongside your other tests, right?;)