I do think it helps to talk to the child ahead of time--but what you say depends on the child. Some kids probably need to be told very firmly that the test is important and they need to do their best, others (who might be anxious) might need to be told that the test is no big deal and not to worry about it.

My DS is one who acts silly, isn't very motivated, and he needs some bribery--i.e. if you work hard on the test I will take you out to lunch after this. He needs that during therapy sessions too since he can get very silly and immature and doesn't always follow directions. He does not like being "put on the spot" being told to perform certain things, and acts out.