I’m not sure the kind of test you may have in mind; nevertheless, there are a variety of tests that could be administered to mentally challenged kids, all depends on your discretion. Your focus shouldn’t be the type of test to practice; the focus should be the probability that the lesson learned over time is truly grabbed. Some learn visually; in this case, tests may be administered pictorially or perhaps via reading/seeing.
Some are auditory learners; i.e., they most preferably learn by listening, tests, in this case, maybe administered verbally. Others are kinesthetic learners in a sense they learn by moving and doing things. The bottom line is that once you are sure of your teaching mechanisms, i.e., they understood properly what they are being taught either via auditory means, visual means or kinesthetics; you wouldn’t have to worry about the rightness or wrongness of a test to practice in as much it is in the confines of their syllabus.