lntest: add HtlcModifier support to node RPC harness

This commit enhances the itest LND node harness to include support for
the new `HtlcModifier` RPC endpoint.
At the same time we move another method to the correct file.

lntest/rpc/router.go

@@ -267,3 +267,19 @@ func (h *HarnessRPC) TrackPayments(
 
 	return resp
 }
+
+type TrackPaymentClient routerrpc.Router_TrackPaymentV2Client
+
+// TrackPaymentV2 creates a subscription client for given invoice and
+// asserts its creation.
+func (h *HarnessRPC) TrackPaymentV2(payHash []byte) TrackPaymentClient {
+	req := &routerrpc.TrackPaymentRequest{PaymentHash: payHash}
+
+	// TrackPaymentV2 needs to have the context alive for the entire test
+	// case as the returned client will be used for send and receive events
+	// stream. Thus we use runCtx here instead of a timeout context.
+	client, err := h.Router.TrackPaymentV2(h.runCtx, req)
+	h.NoError(err, "TrackPaymentV2")
+
+	return client
+}