The problem is that this system will gradually mean that people who get to vote entrench their privilege and keep out others who do not get to vote. Gradually, you'll have a permanent underclass of people who have never known voting. The effects of unequal schooling etc will be even worse than they are today and politicians will have no incentive to even consider people who can't vote.
I really should dredge out my old comment because I already address this concern. Curve the test so that 80% pass. In this way it is impossible to get a feedback loop that miniaturizes the electorate.
