To check if it failsafes, power up the robot (wheels off the ground) push the throttle stick forwards, hold it there, turn of the transmitter, the wheels should stop, likewise for the steering channel.

You need a link, you can't just disconnect the batteries. The link is basically a removable part of the circuit so it cant power up even with batteries connected without the link being in. Think Robo challenge do a ready made Deans one, but you can make one easily enough.