Each ForkReap call started a reap.ReapChildren goroutine that never
stopped (done=nil). Goroutines accumulated across subtests, racing to
call Wait4(-1, WNOHANG) and stealing the child's wait status before
ForkReap's Wait4(pid) could collect it.
Add a WithDone option to pass the done channel through to ReapChildren,
and use it in tests via a withDone(t) helper.