In most places, citizens below a certain age can’t vote, yet laws affect them as well. By extension, one could probably argue that some people “don’t know what’s best for them” and experts/educated people are better suited to make the laws.

(However, creating such a test would obviously be impossible in practice, and would result in a conflict of interest, leading to discrimination, as muusemuuse points out.)

If there were a practical way to do it, a way to ensure that only those who were well informed on a topic could have a say in it wouldn’t be an issue. The only barrier to voting would be your desire to inform yourself.

Unfortunately there isn’t, because just about every word in the above sentences can be twisted by someone with illintent.
The concept isn’t fundamentally flawed, it’s just blocked by insurmountable obstacles.