Once again-the marshmallow test is not a pass/fail test. There is no right answer. It was designed to see what mechanisms children use to delay gratification but specifically does not assert that delaying gratification or not delaying gratification is better. While subsequent studies show a correlation between ability to delay gratification and pursuit of higher education, that does not mean this this is in any way preferable to not delaying gratification.
(As one of the original subjects-I feel that I have the authority to have the definitive answer on this. No-I have no idea how I performed on this experiment but I do blame the researchers for equating sweets with a reward and contributing to a lifetime of weight issues).