The entire idea of the edibility test is testing a plant step by step. After each step, one has to watch and wait for an adverse reaction. The process is a rather lengthy one and in a survival situation, you probably won’t have that kind of time.
The step you need to be concerned with is the last one, and that is to swallow a small piece of the plant and wait for a few hours to see if you see any ill effects. The way to figure out which plants you should eat and which ones to stay away from is by learning about them beforehand.