routing/router: extract sessionKey generation

We extract the session key such that we can later store it across
restarts.

routing/router_test.go

@@ -2442,8 +2442,9 @@ func TestIsStaleEdgePolicy(t *testing.T) {
 func TestEmptyRoutesGenerateSphinxPacket(t *testing.T) {
 	t.Parallel()
 
+	sessionKey, _ := btcec.NewPrivateKey(btcec.S256())
 	emptyRoute := &route.Route{}
-	_, _, err := generateSphinxPacket(emptyRoute, testHash[:])
+	_, _, err := generateSphinxPacket(emptyRoute, testHash[:], sessionKey)
 	if err != route.ErrNoRouteHopsProvided {
 		t.Fatalf("expected empty hops error: instead got: %v", err)
 	}