You can refuse to test if testing would prove harmful or hazardous to yourself or other people . See under abort / abandon test. Its in there somewhere.
Just a quick question , I have been a tester now for 20 years and wanted to know is there anylaws in place to enable you to refuse to test a car that has been presented by a customer who is a heavy smoker? as i frequently get this where the insude is full of ash, smoke and stinks , is there anything in the new smoking laws to protect us nonsmoking testers.