Point of tests is to verify state of the pits after long storage. Plutonium is a bitch to manage because it is alpha-active and alpha particles are in essence, helium atoms, so over time a piece of plutonium gets full of helium caverns and develops internal tension. It's also very complex in terms of crystallic properties (has several stable crystal forms), which are impacted by temperatures and yes, those internal tensions. While it is modelled as much as possible, no one truly knows how good are the pits that were kept in storage for decades, anymore. They certainly still work, but whether they are good enough to properly initiate a secondary, no one really knows. Testing could be very instrumental in finding that out.