This very much depends on which tests we are talking about. Lets consider the IQ tests first, ie verbal and non-verbal reasoning. The purpose of practice is not to increase a child's reasoning ability. Reasoning ability cannot be increased by working through practice tests. What practice can achieve is to make it more likely that a child will perform at his best when taking the selection test. A child will normally continue to improve up to about twelve practice tests although the optimal number for each child will clearly vary.
Practicing English and Mathematics is quite different. These subjects can be 'taught' and English and Mathematics tests 'teach' children as well as 'test' them.