Add test for saving then reading back trade taxes

This test verfies that saved (InvoiceDescriptor.Save()) tax values (ApplicableTradeTax) are also available when read back (InvoiceDescriptor.Load()).
The Test currently fails for the property 'Tax.AllowanceChargeBasisAmount'!

Author: amroel

ZUGFeRD.Test/TestTaxes.cs

@@ -0,0 +1,54 @@
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace s2industries.ZUGFeRD.Test;
+
+[TestClass]
+public class TestTaxes
+{
+    [TestMethod]
+    public void SavingThenReadingAppliedTradeTaxesShouldWork()
+    {
+        InvoiceDescriptor expected = InvoiceDescriptor.CreateInvoice("471102", new DateTime(2013, 6, 5), CurrencyCodes.EUR, "GE2020211-471102");
+        expected.BusinessProcess = string.Empty;
+        expected.Name = string.Empty;
+        expected.ReferenceOrderNo = string.Empty;
+        var lineItem = expected.AddTradeLineItem(name: "Something",
+            grossUnitPrice: 9.9m,
+            netUnitPrice: 9.9m,
+            billedQuantity: 20m,
+            taxType: TaxTypes.VAT,
+            categoryCode: TaxCategoryCodes.S,
+            taxPercent: 19m
+            );
+        lineItem.LineTotalAmount = 198m; // 20 * 9.9
+        expected.AddApplicableTradeTax(
+            basisAmount: lineItem.LineTotalAmount!.Value,
+            percent: 19m,
+            taxAmount: 29.82m, // 19% of 198
+            typeCode: TaxTypes.VAT,
+            categoryCode: TaxCategoryCodes.S,
+            allowanceChargeBasisAmount: -5m
+            );
+        expected.LineTotalAmount = 198m;
+        expected.TaxBasisAmount = 198m;
+        expected.TaxTotalAmount = 29.82m;
+        expected.GrandTotalAmount = 198m + 29.82m;
+        expected.DuePayableAmount = expected.GrandTotalAmount;
+
+        using MemoryStream ms = new();
+        expected.Save(ms);
+        ms.Seek(0, SeekOrigin.Begin);
+
+        InvoiceDescriptor actual = InvoiceDescriptor.Load(ms);
+
+        Assert.AreEqual(expected.Taxes.Count, actual.Taxes.Count);
+        Assert.AreEqual(1, actual.Taxes.Count);
+        Tax actualTax = actual.Taxes[0];
+        Assert.AreEqual(198m, actualTax.BasisAmount);
+        Assert.AreEqual(19m, actualTax.Percent);
+        Assert.AreEqual(29.82m, actualTax.TaxAmount);
+        Assert.AreEqual(TaxTypes.VAT, actualTax.TypeCode);
+        Assert.AreEqual(TaxCategoryCodes.S, actualTax.CategoryCode);
+        Assert.AreEqual(-5m, actualTax.AllowanceChargeBasisAmount);
+    }
+}