I don't have experience of the NHS but I should imagine the comprehension test will give you some text to read (i.e. a letter or a report) and then ask you questions. It's just to check that you can read information and accurately understand the information. I'm not sure if it's something that you can practice.
Take a deep breath, work as quickly as you can without rushing. It is nerve wracking for everyone so don't be too hard on yourself.