The commercial pilot began his takeoff roll on a maintenance test flight. During the ground roll and near rotation speed, the pilot moved his hand from the throttle to the gear handle. The pilot reported that the throttle friction lock was not properly set and the throttle experienced "throttle creep" reducing engine power. The pilot advanced the throttle at which time the airplane became airborne and began to torque roll. An aborted takeoff was executed by the pilot and the airplane touched down off of the runway. The airplane came to rest after contacting the airfield perimeter fence. Substantial damage was sustained to the right wing and left horizontal stabilizer.