Hi Human,
That is terrible! Do you have proof that the test is originally yours (perhaps you emailed it to him and thus you have evidence that it is dated prior to his modification of your text)? If you do have evidence, then you have every right to lodge a complaint against his unethical behavior. I am not sure though if it is acceptable by the University if he has indeed modified it significantly. I know I am not being much help, but I do feel for you! Good luck and do keep us updated on the issue!