It's common practice for a car dealer to put a car through its MOT test at the point of sale. It ensures that the buyer gets a full 12 months of validity. Otherwise, if he had the car tested when he first offered it for sale, any offer of '12 months MOT' would actually be misleading because some time would have almost certainly passed before the car was sold. (i.e. getting the car tested at the point of sale helps to ensure that he's complying with the Trade Descriptions Act, rather than showing that he's operating outside it).
As long as the dealer accepts that the contract to purchase the vehicle is conditional upon the car getting through the test (i.e. he guarantees to refund the money if it fails to do so) he's acting honestly.
If in doubt, wave good old-fashioned folding money under the nose of the dealer and say 'Let's get this car tested right now. If it passes, all this lovely loot will be yours". That way he knows you're serious about buying the car (so he won't be worried about paying for a test and then not getting a sale) but you won't actually part with any money until you know the car has a full MOT.
Chris