itest: add basic invoice HTLC modifier integration test

This commit introduces a basic integration test for the invoice
HTLC modifier. The test covers scenarios where an invoice is settled with a
payment that is less than the invoice amount, facilitated by the invoice
HTLC modifier.

itest/list_on_test.go

@@ -466,6 +466,10 @@ var allTestCases = []*lntest.TestCase{
 		Name:     "forward interceptor restart",
 		TestFunc: testForwardInterceptorRestart,
 	},
+	{
+		Name:     "invoice HTLC modifier basic",
+		TestFunc: testInvoiceHtlcModifierBasic,
+	},
 	{
 		Name:     "zero conf channel open",
 		TestFunc: testZeroConfChannelOpen,