
Putting restrictions like that only invites corruption. What starts as a limit of 100 or 110 eventually becomes 120, then 130, and eventually the limit is 200 and, miraculously, the rich and powerful all manage to score highly (by paying for it of course) and leave everyone else unrepresented.
That was just an idea; there could be others. With advances in technology, tests can be made fair, and scores stored on distributed ledgers, for example. Yes, there may still be corruption; it can be reduced, not eliminated. Also, representation for “everyone” and not “all qualified people” is the root cause. We cannot use the lowest common denominator as a benchmark.
But basing someone's ability to vote on a subjective quality is inherently unfair. Qualifications like "American citizen" or "18 or older" are objective, as in you are definitively one or the other. There's no opinion on whether or not someone is 18 or older or a citizen, just the fact that they are (or aren't). Subjective qualities like intelligence are not a good reference point because there is so much gray area there. Someone who's smart to me might be stupid to you, for example.