>Some may think no CS graduate would fail a test that let you choose any popular language to implement some basic string parsing.
I don't see anything wrong with basic screeners like that, but I have experienced some really silly stuff. Recently I was given a test that asked ~10 questions (some trivia, some paragraph type response), a few small simple questions (reverse a string, etc.), create a class to handle card games (poker, blackjack, etc.), and create a user repository... in 30 minutes. I felt like a dog at a dog show.
Some tests will seem impossible to complete in the alotted time. Often that's intentional. It saves time for the candidates and gives a better spread of scores (very few will get 100% in some test batteries). But it can be disheartening. For these initial online tests I try to give enough time that most people won't really run out of time unless they get stuck.