Who are the new generations that Satan will test after the thousand years?
They are the children born during the thousand-year reign to the believers who entered it at the beginning. These new generations never had to make a choice, so Satan is released at the end to test them and prove the innate rebelliousness of the Adamic nature.
Les explains that after Christ has poured out His wrath, returned, set up His glorious kingdom, and brought in the resurrected of all the ages to rule and reign with Him, He rules over a whole new generation of human beings repopulating the planet. For a thousand years there is perfect rule with no poverty, war, depressions, or anything amiss. The new generations are the children born to the believers who came in at the front end and who never had to make a choice. Satan must therefore be released for the specific purpose of testing these new generations who had never made a choice, and they will again follow Satan.