public void addToStock_Test()
        {
            var product = new Product(5, "prod1", 10, "kg", 0.3);

            itemsInStock.addToStock(product);

            var available     = itemsInStock.isAvailable(5);
            var productResult = itemsInStock.getProduct(5);

            Assert.AreEqual(product, productResult);
            Assert.AreEqual(available, true);
        }