I think it’s your use of the word “test” that is confusing. Using “test” like that makes it sound like some kind of experiment to determine whether or not a plant can experience suffering, and it wouldn’t be a good experiment because it is circular reasoning.

Anyone would choose to stab the plant because they already know that plants are not sentient and do not suffer. Since the choice is made using prior knowledge, the “test” doesn’t determine anything. It is a logical fallacy known as circular reasoning, where the reasoner begins with what they are trying to end with: “plants don’t suffer because when given the choice between stabbing a dog a plant, everyone would choose the plant because plants don’t suffer”.