Testing against your success criteria
A product is only finished when it does what the client needed, and you have to show that it does. That is what testing is for. It is not a tidy-up at the end. It runs alongside development from the first working feature, and it is judged against the success criteria you agreed with the client at the start. You set your success criteria in Criterion A of the documentation, and your IA guide's Criterion A page covers...