Nope. The UFC doesn't actually do the testing; it's the various state athletic commissions. The UFC has no control over that. They test every main-event fighter, plus several others, each event. The UFC has conducted their own tests when there was no athletic commission, which is how Leben got caught.
After devoting a special to how great his conditioning was, Sherk getting caught really put some egg on their face. Especially since he was holding the LW title at the time. Having a stalwart like Leben caught was also embarassing.
As for Brock, he's been tested every time he's fought so far, plus he was tested back when he was in the NCAA, doing amateur wrestling. He's never popped for steroids. He's just a big dude; I don't understand why people just can't accept that.