I don't see how it could work. How could you devise a test that didn't discriminate against people with learning disabilities (I say that because it's difficult to devise a test, btw, not because I mean to imply people with learning disabilities would be naturally worse parents!!).
I think you'd be buggered by the human rights laws, too.
And wouldn't people who come from naice, affluent, non-abusive backgrounds be at a huge advantage, because surely a huge amount of parenting is learned from your parents?
Wouldn't it be better to have mandatory intensive support for everyone who has a baby (impractical I know, esp. looking at the rap HVs get)?