All teas can in theory be viewed as Diet teas, but I am not sure there is any tea I would recommend for fasting. In my experience tea on an empty stomach is a bad idea, and a lot of tea on an empty stomach is far worse.
But all C. Sinensis teas contain nutrients and other components which do help boost metabolism ( I've heard all sorts of numbers, but none are life changing, its something along the lines of 2-15 Calories a day
). Possibly teas best use for a diet, and this again is true of all teas, is as long as you are not adding milk, or sweeteners, tea is basically a zero Calorie beverage, and chances are it will replace other beverages, and if it replaces things like Soda's or Fruit Juices, you are dieting because you are cutting those Calories from your diet.