create promotion_test.js to test Promotion.all()
create util_test.js file to test Util.toFixed()
add scanner_test.js to test Class of Scanner
add promotion_calculator_test to test PromotionCalculator
test getInventoryText() in cart_test.js
test getPromotionText() & addCartItem() in cart_test
create describe of getTotalAmount() to test cart.getTotalAmount()
create describe of getSavingMoney() to test cart.getSav
add test tostring method in cart_item.js