public void Can_Get_TotalTax()
{
var purchaseOrderSut = new SalesOrder();
purchaseOrderSut.AddOrderLine(new OrderLineStubifier().GivenTax(5m).StubIt());
purchaseOrderSut.AddOrderLine(new OrderLineStubifier().GivenTax(1m).StubIt());
var totalTax = purchaseOrderSut.TotalTax();
Assert.AreEqual(5m + 1m, totalTax);
}