During the runup I firmly put my feet on the brakes and squeeze my knees together to hold the stick full aft. I then have both hands free to perform the runup - one hand on the throttle and the other free to cycle the magnetos and the carb heat. The same for starting the engine by the way.
I'm surprised that instructors/examiners allow their students/examinees to perform power checks or engine starts on parking brakes alone. The parking brake is just that - a brake used when parking the aircraft.
It's especially bad practice for a night VFR/IFR flight as the visual cues that the aircraft is moving unintentionally are far less.