Add Order and OrderBook domains with test suites

csharp/tests/OrderTests/OrderCompletionTests.cs

@@ -0,0 +1,17 @@
+using NUnit.Framework;
+using OrderDomain;
+
+namespace OrderTests;
+
+public class OrderCompletionTests
+{
+    [Test]
+    public void CompletedOrder_TotalIsCorrect()
+    {
+        var order = new Order();
+        order.AddLine(100m, 1);
+        order.Complete();
+
+        Assert.That(order.Total, Is.EqualTo(100m));
+    }
+}