As much as I dislike bad parents and unruly children, I think this is a bad idea. A test like this fails in its subjectivity. What is good parenting? Apart from the 'obvious feed child, clothe child, take child to doctor if sick', what would the test be based on? Methods of raising children? There are _thousands_ of different methods and philosophies behind raising children. Which one would be taught? Which one is right? Who decides which one is right? Kids are not sprung from a cookie cutter, different methods work differently on different children.
Also, as has been pointed out, what happens if a prospective parent fails? Are they castrated/receive a vasectomy so as they'll never be able to have children, since they've been deemed unfit by society? What about rape victims that are impregnated? What about religious groups that are against abortion?
Or should we do like China have with the one-child policy - let people send in applications to have children, and if they are pregnant without the a-ok from the state, they're fined?
No. Bad people will be bad people, and yes, some kids will be unlucky enough to be born to these individuals. But to make society as a whole go through a test to conceive and do something that's natural and instinctive is rather sickening to me.