release 3.9.0 process: reduce warnings (#279)

* fix(riseupvpn): address gofmt warning

Thanks to https://goreportcard.com/report/github.com/ooni/probe-cli.

* fix(utils.go): correct the docu-comment

Thanks to https://goreportcard.com/report/github.com/ooni/probe-cli

* fix: improve spelling

Thanks to https://goreportcard.com/report/github.com/ooni/probe-cli

* fix(modelx_test.go): avoid inefassign warning

Thanks to https://goreportcard.com/report/github.com/ooni/probe-cli

* fix: reduce number of ineffective assignments

Thanks to https://goreportcard.com/report/github.com/ooni/probe-cli

internal/engine/netx/errorx/errorx_test.go

@@ -175,6 +175,9 @@ func TestToFailureString(t *testing.T) {
 		defer cancel() // fail immediately
 		udpAddr := &net.UDPAddr{IP: net.ParseIP("216.58.212.164"), Port: 80, Zone: ""}
 		udpConn, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4zero, Port: 0})
+		if err != nil {
+			t.Fatal(err)
+		}
 		sess, err := quic.DialEarlyContext(ctx, udpConn, udpAddr, "google.com:80", &tls.Config{}, &quic.Config{})
 		if err == nil {
 			t.Fatal("expected an error here")