Oh man, I love Testament!!!
I've seen Skolnick a million times, he lives very close to my house.
Anyway, I would say that tone is easy with:
Ibanez > Nailbomb > Marshall stack
The older stuff has a fair amount of 'verb, and is also compressed. The mids are slighly scooped.
PS- do yourself a favour and get First Strike Still Deadly.
Edit #2; on some stuff, Skolnick also used a hollow bodied Ibie.