Cauliflower soup is something that could be incredibly boring — but it certainly doesn’t have to be. This thoughtful recipe from “America’s Test Kitchen” cooks the vegetable two different ways to attain maximum flavor mileage. Cauliflower Soup Serves 4 to 6 1 head cauliflower (2 pounds) 8 tablespoons unsalted butter, cut into 8 pieces 1…