hltcswitch/hop: add next hop to intermediate hop with custom records

Fix our existing test to have a valid intermediate hop that will pass
stricter validation. Previously, we did not specify a next channel for
an intermediate hop (which violates bolt4).

htlcswitch/hop/payload_test.go

@@ -168,7 +168,15 @@ var decodePayloadTests = []decodePayloadTest{
 	{
 		name:       "required type in custom range",
 		isFinalHop: false,
-		payload: []byte{0x02, 0x00, 0x04, 0x00,
+		payload: []byte{
+			// amount
+			0x02, 0x00,
+			// cltv
+			0x04, 0x00,
+			// next hop id
+			0x06, 0x08,
+			0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			// custom
 			0xfe, 0x00, 0x01, 0x00, 0x00, 0x02, 0x10, 0x11,
 		},
 		expCustomRecords: map[uint64][]byte{