The President (and congress) should know what is in the laws (or executive orders) they are creating or signing. Forcing them to pass a test (85% or better) will ensure they at least know what is in the bill. A simple bill may only require a few questions. A complex bill like the Affordable Care Act may require 200-300 questions. Questions would be written by GAO.
Doesn't this create an easy way to avoid voting on a bill? Good question. I suspect that voters would not put up with someone who isn't doing their job (I recognize that the most recent election does not reflect this sentiment).