delete private function test
add

should_return_second_half_price_promotion_list test
finish calculateSecondHalf method and test pass
finish hasExistDiscount method and test pass
add hasExistOneFree method and test
finish  hasExistOneFree method and test
add hasExistSecondHalf method and test
finish hasExistSecondHalf method and test
modify public function to private function and delete tests
modify PromotionCalculator.java
add getLowestPrice function and test
finish getLowestPrice function and test
format codes