Or, alternately, they know that they will feel better after the test if the test tells them that they have a healthier heart, so they act in such a way as to get that internal reward.

Human reward mechanisms just aren't set up properly; in this case, the internal reward isn't actually for living longer, it's for passing the test, so you try to pass the test. In some ways, it's similar to a standardized test in school; your actual aptitude or intelligence doesn't change based on how hard you try on the test, but you try hard anyway, because your reward mechanisms aren't based on your intelligence per se, they're based on the test score, so that's what you try to maximize.