Why? Are you a bad driver? 😉
Ah, but seriously, if you've had enough real world practice that you can drive around town comfortably, without cutting anyone off or maiming pedestrians and bikers, and you know all the laws, you'll be fine.
The big things to know (for testing AND real driving):
Know what to do at 4 way stops and uncontrolled intersections. (Yield to any vehicle to YOUR right--and slow way down at uncontrolled intersections.)
Know what to do at malfunctioning traffic lights (Flashing red = stop sign. Flashing yellow = procede with caution. No lights, or power failure = ALL WAY STOP SIGN.)
Know when it's OK to turn left on red. (From a one way onto another one way ONLY. Never onto, or from, a two way street.)
Relax. If you can drive the car smoothly, and you don't mess up the above (and don't speed or do anything stupid), you'll do fine. 🙂
EDIT: And if you don't do fine, that's OK too. Don't feel bad if you need more practice. With only six hours, we won't! 😉