refactor(netx): move dns transports in netxlite/dnsx (#503)

While there, modernize the way in which we run tests to avoid
depending on the fake files scattered around the tree and to
use some well defined mock structures instead.

Part of https://github.com/ooni/probe/issues/1591

internal/netxlite/dnsx/serial_test.go

@@ -0,0 +1,148 @@
+package dnsx
+
+import (
+	"context"
+	"crypto/tls"
+	"errors"
+	"net"
+	"strings"
+	"testing"
+
+	"github.com/miekg/dns"
+	"github.com/ooni/probe-cli/v3/internal/netxlite/dnsx/mocks"
+	"github.com/ooni/probe-cli/v3/internal/netxlite/errorsx"
+)
+
+func TestOONIGettingTransport(t *testing.T) {
+	txp := NewDNSOverTLS((&tls.Dialer{}).DialContext, "8.8.8.8:853")
+	r := NewSerialResolver(txp)
+	rtx := r.Transport()
+	if rtx.Network() != "dot" || rtx.Address() != "8.8.8.8:853" {
+		t.Fatal("not the transport we expected")
+	}
+	if r.Network() != rtx.Network() {
+		t.Fatal("invalid network seen from the resolver")
+	}
+	if r.Address() != rtx.Address() {
+		t.Fatal("invalid address seen from the resolver")
+	}
+}
+
+func TestOONIEncodeError(t *testing.T) {
+	mocked := errors.New("mocked error")
+	txp := NewDNSOverTLS((&tls.Dialer{}).DialContext, "8.8.8.8:853")
+	r := SerialResolver{
+		Encoder: &mocks.Encoder{
+			MockEncode: func(domain string, qtype uint16, padding bool) ([]byte, error) {
+				return nil, mocked
+			},
+		},
+		Txp: txp,
+	}
+	addrs, err := r.LookupHost(context.Background(), "www.gogle.com")
+	if !errors.Is(err, mocked) {
+		t.Fatal("not the error we expected")
+	}
+	if addrs != nil {
+		t.Fatal("expected nil address here")
+	}
+}
+
+func TestOONIRoundTripError(t *testing.T) {
+	mocked := errors.New("mocked error")
+	txp := &mocks.RoundTripper{
+		MockRoundTrip: func(ctx context.Context, query []byte) (reply []byte, err error) {
+			return nil, mocked
+		},
+		MockRequiresPadding: func() bool {
+			return true
+		},
+	}
+	r := NewSerialResolver(txp)
+	addrs, err := r.LookupHost(context.Background(), "www.gogle.com")
+	if !errors.Is(err, mocked) {
+		t.Fatal("not the error we expected")
+	}
+	if addrs != nil {
+		t.Fatal("expected nil address here")
+	}
+}
+
+func TestOONIWithEmptyReply(t *testing.T) {
+	txp := &mocks.RoundTripper{
+		MockRoundTrip: func(ctx context.Context, query []byte) (reply []byte, err error) {
+			return genReplySuccess(t, dns.TypeA), nil
+		},
+		MockRequiresPadding: func() bool {
+			return true
+		},
+	}
+	r := NewSerialResolver(txp)
+	addrs, err := r.LookupHost(context.Background(), "www.gogle.com")
+	if err == nil || !strings.HasSuffix(err.Error(), "no response returned") {
+		t.Fatal("not the error we expected")
+	}
+	if addrs != nil {
+		t.Fatal("expected nil address here")
+	}
+}
+
+func TestOONIWithAReply(t *testing.T) {
+	txp := &mocks.RoundTripper{
+		MockRoundTrip: func(ctx context.Context, query []byte) (reply []byte, err error) {
+			return genReplySuccess(t, dns.TypeA, "8.8.8.8"), nil
+		},
+		MockRequiresPadding: func() bool {
+			return true
+		},
+	}
+	r := NewSerialResolver(txp)
+	addrs, err := r.LookupHost(context.Background(), "www.gogle.com")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(addrs) != 1 || addrs[0] != "8.8.8.8" {
+		t.Fatal("not the result we expected")
+	}
+}
+
+func TestOONIWithAAAAReply(t *testing.T) {
+	txp := &mocks.RoundTripper{
+		MockRoundTrip: func(ctx context.Context, query []byte) (reply []byte, err error) {
+			return genReplySuccess(t, dns.TypeAAAA, "::1"), nil
+		},
+		MockRequiresPadding: func() bool {
+			return true
+		},
+	}
+	r := NewSerialResolver(txp)
+	addrs, err := r.LookupHost(context.Background(), "www.gogle.com")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(addrs) != 1 || addrs[0] != "::1" {
+		t.Fatal("not the result we expected")
+	}
+}
+
+func TestOONIWithTimeout(t *testing.T) {
+	txp := &mocks.RoundTripper{
+		MockRoundTrip: func(ctx context.Context, query []byte) (reply []byte, err error) {
+			return nil, &net.OpError{Err: errorsx.ETIMEDOUT, Op: "dial"}
+		},
+		MockRequiresPadding: func() bool {
+			return true
+		},
+	}
+	r := NewSerialResolver(txp)
+	addrs, err := r.LookupHost(context.Background(), "www.gogle.com")
+	if !errors.Is(err, errorsx.ETIMEDOUT) {
+		t.Fatal("not the error we expected")
+	}
+	if addrs != nil {
+		t.Fatal("expected nil address here")
+	}
+	if r.NumTimeouts.Load() <= 0 {
+		t.Fatal("we didn't actually take the timeouts")
+	}
+}