Remove basket items when basket is deleted (#170)

* Proving out removing the basket items at the same time the basket is deleted

* Fixing existing unit tests

* Adding a unit test for the Deletebasket method

* Rename test and deleting basket items before basket

* Added integration test for DeleteAsync method for BasketItems

- Also added a BasketBuilder to create a Basket with no items, or one item.

tests/UnitTests/Builders/BasketBuilder.cs

@@ -0,0 +1,34 @@
+using Microsoft.eShopWeb.ApplicationCore.Entities.BasketAggregate;
+
+namespace Microsoft.eShopWeb.UnitTests.Builders
+{
+    public class BasketBuilder
+    {
+        private Basket _basket;
+        public string BasketBuyerId => "testbuyerId@test.com";
+        public int BasketId => 1;
+
+        public BasketBuilder()
+        {
+            _basket = WithNoItems();
+        }
+
+        public Basket Build()
+        {
+            return _basket;
+        }
+
+        public Basket WithNoItems()
+        {
+            _basket = new Basket { BuyerId = BasketBuyerId, Id = BasketId };
+            return _basket;
+        }
+
+        public Basket WithOneBasketItem()
+        {
+            _basket = new Basket { BuyerId = BasketBuyerId, Id = BasketId };
+            _basket.AddItem(2, 3.40m, 4);
+            return _basket;
+        }
+    }
+}