public void AddTailTest()
        {
            // Arrange
            var list = new SimpleLinkedList <int>();

            list.AddTail(1);
            list.AddTail(2);
            list.AddTail(3);
            list.AddTail(4);

            // Act => Assert
            list.Head.Should().Be(1);
            list.Tail.Should().Be(4);
            list.Count.Should().Be(4);
        }