Matt they test globally. They do Colorado for high altitude testing. Death Valley for hot testing. Canada and northern Michigan for cold and Germany for suspension calibration.
They also do a lot of road testing in southern Ohio.
Each site offers different conditions that customers will find depending on where they live.
GM tested the 1984 C4 only at the Mesa test track and Michigan test track and the Z51 suspension was horrid on rough roads because it never really saw real world roads till the press got their hands on it. They found the shocks were too stiff and replaced them for 1985.