Literary tests didn't violate the fifteenth amendment because people had to understand what they were selecting. It also allowed people to communicate with others easier. Without being able to read and write the news, text on the news, and books were inaccessible to whoever needed the information from those mediums.
Supreme Court hearings in the 1950's found that literary tests were in fact in violation of the fifteenth amendment. This was a problem in the south where blacks were required to take literary tests that whites were grand-fathered into. The Supreme Court made a very clear decision on this topic and it should not be a matter of debate at this point.
Literacy tests were definitely a violation of the fifteenth amendment. While the test did not directly state "if you are not white, you cannot vote," but the vast disparity between the schooling of blacks and white would mean that black were not able to read. Literacy tests targeted them directly.
No, policies like literary tests are not in violation of the 15th Amendment, because there are good reason to have these things. A literary test is a good idea to make sure that person knows what they are voting for. It is not meant to be racist. We have to still be able to pass good laws.