No. Positive selection means taking anyone who performs exceptionally well on any one out of many possible measures, whereas negative selection means taking anyone who performs at least passably on all of many measures. On the scale of how to integrate the information you get from each test, there only is one test, so you can't distinguish between positive and negative selection.

On the scale of what the test itself measures, most exams tend to provide negative selection because the students usually know most of the material, so the smart kids can reliably get more than 90% of the questions correct, and the best way to do well involves having as few weak points as possible, rather than having some exceptionally strong abilities. So unless the one massive examination that they take in China is the Putnam, it's probably more like negative selection.