public void NotFindPairs(int[] arr, int sum)
        {
            var sut = new PairFinderForSpecificSum();

            var result = sut.FindPair(arr, sum);

            result.Should().BeNull();
        }
        public void FindPairs(int[] arr, int sum, int expectedFirstElement, int expectedSecondElement)
        {
            var sut = new PairFinderForSpecificSum();

            var result = sut.FindPair(arr, sum);

            result.Should().NotBeNull();
            result.FirstElementIndex.Should().Be(expectedFirstElement);
            result.SecondElementIndex.Should().Be(expectedSecondElement);
        }