Testament didn't remaster them, they re-recorded them.
That's one of the things I like about recent (well, ish) greatest hits albums by Godflesh and Napalm Death, they took a lot of the older material and remastered it, so it sounds great. Especially good in the case of tracks off of Godflesh's Streetcleaner album, since the volume levels and mixing on the original release were horrible.
I still think Anthrax need to remaster Among The Living as well.