Welcome to the forum
I do think that you should return the car back to them
advising them that you have been charged for repairs that have not been done and the problem remains.
Don't pay for another test, it should be up to them to detect the fault and advise you. try to recover some of your money you paid them it's not a good garage that cannot fault find. if needs be take it to another garage and then pass the bill onto the old garage.