While on test the examiner will expect you to drive according to the weather conditions. If the weather becomes too extreme your test will be cancelled. Should this happen you will be offered an alternative date at no extra charge to yourself. You will also be able to claim back the hire of the car. Your instructor will be able to advise you on that.
You should make allowances for the weather in your driving. This generally means that in fog you should be able to stop in the distance that you can see is safe in front of you. If the road surface condition requires more stopping room due to rain, snow or ice. Make sure you give enough room.
You will also be expected to make suitable adjustments with the car. See and be seen. This is being able to put your lights on and keep your windows clear. Know your controls and be able to operate them.
Remember that this can include glare from the sun. Use the sun visor if appropriate and practice while driving. Have some sunglasses handy if needed.