invoices: enhance the unit test suite.

The invoiceregistry test suite also includes unit tests for
multi part payment especially also including payments to AMP
invoices.

invoices/test_utils_test.go

@@ -207,7 +207,7 @@ func getCircuitKey(htlcID uint64) invpkg.CircuitKey {
 // Note that this invoice *does not* have a payment address set. It will
 // create a regular invoice with a preimage is hodl is false, and a hodl
 // invoice with no preimage otherwise.
-func newInvoice(t *testing.T, hodl bool) *invpkg.Invoice {
+func newInvoice(t *testing.T, hodl bool, ampInvoice bool) *invpkg.Invoice {
 	invoice := &invpkg.Invoice{
 		Terms: invpkg.ContractTerm{
 			Value:    testInvoiceAmount,
@@ -217,6 +217,23 @@ func newInvoice(t *testing.T, hodl bool) *invpkg.Invoice {
 		CreationDate: testInvoiceCreationDate,
 	}
 
+	// This makes the invoice an AMP invoice. We do not support AMP hodl
+	// invoices.
+	if ampInvoice {
+		ampFeature := lnwire.NewRawFeatureVector(
+			lnwire.TLVOnionPayloadOptional,
+			lnwire.PaymentAddrOptional,
+			lnwire.AMPRequired,
+		)
+
+		ampFeatures := lnwire.NewFeatureVector(
+			ampFeature, lnwire.Features,
+		)
+		invoice.Terms.Features = ampFeatures
+
+		return invoice
+	}
+
 	// If creating a hodl invoice, we don't include a preimage.
 	if hodl {
 		invoice.HodlInvoice = true