You don't need Viper's radars unless you are doing a combat simulation. The land based AN/TPS radars being used by Pakistan, have state of the art tech in them. If they fail to detect a plane, I am sure it would be the same for the -16!!! But I have a feeling that the AN/TPS will detect the J-31. May be a couple of dozen miles after its supposed to, due to J-31's stealthy features, but it would capture the signature.
It's because a verbal commitment in principle has been made between both the Premiers that Pakistan will buy 40-44 of these initially. With the option for 20 more later (supporting the naval air arm with dedicated stealth, or securing the ports I'd imagine?).
So now that there is verbal contract in place and the negotiations have advanced, it makes sense for the PAF to test out this plane per their operational needs.