To dream that you are taking a test indicates that you are being put to the test or being scrutinized in some way. If you fail the test, then it suggests that you are feeling inadequate or insecure about some aspect of your waking life. If you ace the test, then it means that you are prepared and ready for the task at hand. There is also the notion that if you visualize success, then you will achieve success in your real life. Thus, this dream may be a “practice exercise” for you to visualize success. Alternatively, the dream may be a pun on “feeling testy” and irritated.
Please see Common Dreams: Exam.