"Extensive testing" means conducting very thorough and comprehensive tests to make sure something is working correctly or to find any potential issues or problems. Full definition
Scientists regularly evaluated the participants for up to seven years, including how they performed in extensive tests of mental function and psychological health. (sciencedaily.com)
But this should only be after extensive testing by qualified psychiatrists, and with continual and careful monitoring. (home-ed.info)
We have adjusted this recipe because of extensive testing on the road. (fieryfoodscentral.com)