while currently in stepper test mode the extend feature works as
intended now.
you can trigger movement using the buttons and repeating
presses while still moving (coast, accel or decel) extends the movement
without stopping or accelerating again.
Note reversing the direction while still moving is not handled and results
in immediate direction change and distance that makes no sense.
also added alot of debug output and switched to debug level
1c59e0097b