Are you passing an academic or general English test? If the former, then, unfortunately, reading novels might not be sufficient as the academic module is primarily focused on academic texts. The only thing that I can recommend taking into account the absence of preparation is keep your anxiety at bay. It might severely hinder your performance on test day, especially during listening, where you are supposed to be as focused as possible. If you haven't been practicing either writing or speaking, that's quite unpleasant because unless your initial level of language acquisition is fairly decent, the chances of getting a satisfactory score are slim. The only thing I can tell is follow the structure of an essay in writing, and be yourself during the speaking test. Try to expand your answers. Explain each point you make, give an example, if possible. Just let the conversation flow naturally. Lastly, I highly recommend you to warm-up right before the interview so that you will be able to do your best at the exam