I don't know why objections to voting tests usually pretend we're in 1850. We have standardized tests, already, nationwide. It's a solvable problem. We wouldn't contingent a vote on a random poll worker's choice of passage to read.
And voting is legislated by individual states, that would theoretically implement their own standards though this may be intervened upon by the federal government). Heck, even standardized testing for students is done at a state level. The SATs/ACTs are privately administered. What example of a nationwide standardized test for literacy do you have?