I don’t think a certifying test is a bad idea. Engineers have one to become a professional engineer. But computer science is so broad that you would need a bunch of them. For example, different ones that focuses topics like web programming, computer graphics or computer security.
For many majors, such a test would make no sense. You could have a certifying test in history, but there are very few jobs that specifically require a knowledge of history.
As I said, for computer science grads there is increasing emphasis on what you can show, not just what you know. So being able to show a program you wrote or what you did on an open source software project can be more important than your GPA. Also, most employers will have a guru test you as part of the interview. All of these are positive trends.
My daughter graduated with a B.S. in mathematics at the age of 18. ( She was homeschooled.) She has made a nice career for herself as a data analyst for an insurance company. He goal is to pass Oracle exams. (Do I have that right? Her work is complete outside anything in my experience.)