itest: add itest for field modification HTLC interception response

Implement an integration test where an HTLC is intercepted and the
interception response modifies fields in the resultant p2p message.

itest/list_on_test.go

@@ -454,6 +454,10 @@ var allTestCases = []*lntest.TestCase{
 		Name:     "forward interceptor",
 		TestFunc: testForwardInterceptorBasic,
 	},
+	{
+		Name:     "forward interceptor modified htlc",
+		TestFunc: testForwardInterceptorModifiedHtlc,
+	},
 	{
 		Name:     "zero conf channel open",
 		TestFunc: testZeroConfChannelOpen,