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/dnsoverudp_test.go

@@ -0,0 +1,157 @@
+package dnsx
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"net"
+	"testing"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/netxlite/mocks"
+)
+
+func TestDNSOverUDPDialFailure(t *testing.T) {
+	mocked := errors.New("mocked error")
+	const address = "9.9.9.9:53"
+	txp := NewDNSOverUDP(&mocks.Dialer{
+		MockDialContext: func(ctx context.Context, network, address string) (net.Conn, error) {
+			return nil, mocked
+		},
+	}, address)
+	data, err := txp.RoundTrip(context.Background(), nil)
+	if !errors.Is(err, mocked) {
+		t.Fatal("not the error we expected")
+	}
+	if data != nil {
+		t.Fatal("expected no response here")
+	}
+}
+
+func TestDNSOverUDPSetDeadlineError(t *testing.T) {
+	mocked := errors.New("mocked error")
+	txp := NewDNSOverUDP(
+		&mocks.Dialer{
+			MockDialContext: func(ctx context.Context, network, address string) (net.Conn, error) {
+				return &mocks.Conn{
+					MockSetDeadline: func(t time.Time) error {
+						return mocked
+					},
+					MockClose: func() error {
+						return nil
+					},
+				}, nil
+			},
+		}, "9.9.9.9:53",
+	)
+	data, err := txp.RoundTrip(context.Background(), nil)
+	if !errors.Is(err, mocked) {
+		t.Fatal("not the error we expected")
+	}
+	if data != nil {
+		t.Fatal("expected no response here")
+	}
+}
+
+func TestDNSOverUDPWriteFailure(t *testing.T) {
+	mocked := errors.New("mocked error")
+	txp := NewDNSOverUDP(
+		&mocks.Dialer{
+			MockDialContext: func(ctx context.Context, network, address string) (net.Conn, error) {
+				return &mocks.Conn{
+					MockSetDeadline: func(t time.Time) error {
+						return nil
+					},
+					MockWrite: func(b []byte) (int, error) {
+						return 0, mocked
+					},
+					MockClose: func() error {
+						return nil
+					},
+				}, nil
+			},
+		}, "9.9.9.9:53",
+	)
+	data, err := txp.RoundTrip(context.Background(), nil)
+	if !errors.Is(err, mocked) {
+		t.Fatal("not the error we expected")
+	}
+	if data != nil {
+		t.Fatal("expected no response here")
+	}
+}
+
+func TestDNSOverUDPReadFailure(t *testing.T) {
+	mocked := errors.New("mocked error")
+	txp := NewDNSOverUDP(
+		&mocks.Dialer{
+			MockDialContext: func(ctx context.Context, network, address string) (net.Conn, error) {
+				return &mocks.Conn{
+					MockSetDeadline: func(t time.Time) error {
+						return nil
+					},
+					MockWrite: func(b []byte) (int, error) {
+						return len(b), nil
+					},
+					MockRead: func(b []byte) (int, error) {
+						return 0, mocked
+					},
+					MockClose: func() error {
+						return nil
+					},
+				}, nil
+			},
+		}, "9.9.9.9:53",
+	)
+	data, err := txp.RoundTrip(context.Background(), nil)
+	if !errors.Is(err, mocked) {
+		t.Fatal("not the error we expected")
+	}
+	if data != nil {
+		t.Fatal("expected no response here")
+	}
+}
+
+func TestDNSOverUDPReadSuccess(t *testing.T) {
+	const expected = 17
+	input := bytes.NewReader(make([]byte, expected))
+	txp := NewDNSOverUDP(
+		&mocks.Dialer{
+			MockDialContext: func(ctx context.Context, network, address string) (net.Conn, error) {
+				return &mocks.Conn{
+					MockSetDeadline: func(t time.Time) error {
+						return nil
+					},
+					MockWrite: func(b []byte) (int, error) {
+						return len(b), nil
+					},
+					MockRead: input.Read,
+					MockClose: func() error {
+						return nil
+					},
+				}, nil
+			},
+		}, "9.9.9.9:53",
+	)
+	data, err := txp.RoundTrip(context.Background(), nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(data) != expected {
+		t.Fatal("expected non nil data")
+	}
+}
+
+func TestDNSOverUDPTransportOK(t *testing.T) {
+	const address = "9.9.9.9:53"
+	txp := NewDNSOverUDP(&net.Dialer{}, address)
+	if txp.RequiresPadding() != false {
+		t.Fatal("invalid RequiresPadding")
+	}
+	if txp.Network() != "udp" {
+		t.Fatal("invalid Network")
+	}
+	if txp.Address() != address {
+		t.Fatal("invalid Address")
+	}
+}