The common cognitive test in the UK is the CATs (Cognitive Attitude Test). It measures the three principal areas of reasoning - verbal, non-verbal and numerical - as well as an element of spatial ability.
It is good for highlighting particular strengths and weaknesses. It can be used to help develop strategies to help your child better access the curiculum.
It is not used by many schools for all their students, and can be used by others to test your childs reasoning skills.
My daughter was required to do this as part of the entry requirement to one of the schools we looked at when she moved to secondary, and some school do invest in it. It may have been used as part of her diagnosis, but don't recall it.
Ask the psychologist for an interview after the test so they can explain the results to you, and how you can use this information to help your child. The test itself is not a major thing, and the best way of preparing your child imo is not to get stressed about it.
What I do recall was a great deal of relief that they were finally seriously looking at why our daughter was facing so many difficulties, and consternation that the school had to try and contact us to get permission before allowing anything beyond observation and the short notice they were given of visits we chose to give them permission to go ahead with these things if they did not have enough time to inform us as long as they did inform us at the earliest possible time.
It can be a very difficult time with a lot happening and very little understanding of what it means or what is said. Proper understanding often comes later. Get yourself a diary or notebook and jot down your questions, things that are said you don't understand. When you get to the end of the appointment ask for another. Just explain that you have been given a lot of information and need time to process it but would like the opportunity to ask more questions when this has been done. We have always found them more then happy to do this and very understanding.
Also ask if your local autism clinic or practioners hold workshops for parents. I have learned a great deal through these.