`nil` value `TestReporter` in `NewController`

[cyrusv]

Is this allowed or not? It does not throw an error right away, but opens the door for NPEs in methods like (h *nopTestHelper) Errorf(. I understand it might be backwards incompat to start enforcing now, so it is a constraint we can relax, or at least document?

[r-hang]

I believe that we should support a nil input but rework the logic of the nopTestHelper to have an actual nop implementation instead of something that would panic.