Then we won't have a democracy, but yes some form of oligarchy where only people with time to study politics (a.k.a. rich people) would have a vote. And the test could be made biased by the governing party to support only their ideology.
This (What we live in) is still a democracy, if only in name.
It's easy to make a standardised test that reveals basic knowledge of politics without having it lean one way or the other. It should not have to be necessary to study for it, only have a basic understanding of politics, actions and reactions. This is not much to demand from a member of a society.
the principle is meant to help society. Society does not necessarily benefit from the greater consensus. Just because everyone has a voice, does not mean every voice is worth listening to. Democracy often fails on this principle. pragmatism must come before principles.
Who decides what the questions are? If it's the elected officials, it's far too likely that they would manipulate the questions in order to secure themselves a seat. You already see similar versions of this with other forms of voter control.