100 above is correct. If you do the test before engine start the APU must be off....you are testing everything apart from the valves (which need to be tested after engine start).
The valves are electrically operated and pnumatically actuated so they need enough pressure to open them and keep them open as well as the eletrical signal commanding them open.
1. The APU bleed air cannot supply this mass airflow
2. If you try to make it you run the risk of an overtemp.