As soon as you enter the test chamber pick up the turret in front of you and drop it down so it falls over. Stay away from its bullets until it closes its panels and then carefully approach the corner.
Head up the nearby stairs and then you should notice two turrets behind a protective glass shield. The trick here is to place one portal directly above each of the turrets and then set the second portal under each of the two Companion Cubes to your right. They’ll land on the turrets, opening the way forward.
Now pick the Cube back up and use it as a battering ram/shield to knock over the remaining two turrets before setting it down on the switch and opening up the exit.