OK. So a test would sort that out and test that thesis, wouldn't it?
To deny all women/blacks/homosexuals/<add your own category of people here> a vote is to claim that there is something inherent about that category of people which means they're not competent to vote.
That is absolutely not the same as saying that in order to vote you should have to demonstrate basic knowledge about what you're voting for. And, again, I'm not talking about an intelligence test. But if someone has no idea who the party leaders are, what they're standing for, what's in their manifesto...on what basis are they voting then?
No-one thinks that "being a human" is the only criteria for being able to vote. The categories of people who can vote has been expanded over time - it used to just be property owners. Even now only people over 18 can vote. Why can't under 18s vote? It can't be true that everyone under 18 is incompetent to. I'm sure a lot of 16 and 17 year olds know more about politics than me. So why can't they? Policies which governments make don't just affect adults. Prisoners can't vote. Non-citizens can't vote - so my German mate who has lived here for getting on about 30 years but has never wanted to have UK citizenship can't vote in a General Election even though he's been living in and paying taxes in this country all this time.
You haven't actually made any counter-argument to my suggestion, you've just incorrectly conflated it with archaic views about whether certain categories of people can vote. Saying "only men should be able to drive" is absolutely not the same as saying "there should be a test before someone can drive".