It is not a big deal if you stall the car on the driving test, but you need to recover the car the correct way. There is no point in panicing, you've already got a driver error for the mistake, and you don't want to accumulate more or get a serious for doing something dangerous if you want to pass the test, so follow the tip given below.
The first thing you should do if your car stalls, is to secure the car from rolling out of control, by applying the handbrake (also known as parking brake). You can now put the car in neutral if necessary (if you stalled because you were trying to move off in 2nd or 3rd gear), otherwise you can press the clutch down and restart the engine if you were in reverse or first gear.
You can now get the car prepared (make sure you have the biting point if you are doing a hill start), do your observations (very important, as cars, motorbikes or cyclists might be trying to get round you), and then move off when it is safe. I know cars around you might be harrassing you, but you've already made the mistake, and you need to keep your cool to avoid falling your test due to the aggression of these other road users (this is why you need to practice this during your driving lessons).
There is no reason why you shouldn't pass your driving test even if you stall your car, I did.
If you stall the car a lot during driving lessons, then you need to find out why, is it because you've just changed to a petrol car which can be very different to diesel, or the vehicle has just been changed by the instructor and the clutch is different, what ever the case is, go somewhere quiet and practise until you get it right.
This will boost your confidence when you get to more busy traffic conditions, and especially on the DSA driving test.
Extra test advice
Passing the test guaranteed by some organisations.