Historically, literacy tests had nothing to do with literacy, they were exclusively for racist purposes, and to subjugate 'undesireables'. They were handed out at the (almost certainly racist) poller's discretion, and the tests themselves were worded so poorly that most of the questions had no single, correct answer. They were not real tests. Comparing literacy tests to the above poster's suggestion is like comparing aspirin to cyanide because they can both be put into a pill and swallowed.
Over the past few years, we've seen numerous examples of the destruction that voters can create in democratic societies. Turkey is probably not a democracy any more, Russia definitely isn't. Brexit is probably due to a campaign that was based on out right lies. The notion that universal suffrage may not be the best system is no longer unreasonable. We certainly have lots of examples by now.
What does it mean in a democracy if people are voting for options that they literally know nothing about?
Here's an example of a voting test that I think would be totally fair. 3-6 months before the election, 20 multiple choice questions, and their answers, are released. The questions are all simple statement of facts. "What is the name of the person currently in such and such position." "What number amendment is quoted here". "If the president and vice president die, what is the name of the person who would assume the role of president." Then, when someone submits a ballot, they're handed 5 of these questions chosen at random, and they have to answer 4 correctly. The questions are all known ahead of time, so if there's a problem with one of them, everyone will know. If someone gets into the polling booth and can't answer these questions, then they don't know the most basic facts of our political system, and they didn't even bother to look them up even though they were explicitly told that they should know them.
Universal suffrage was a huge improvement on the political systems that came before, but that doesn't mean there isn't a better way.