refactor: DNSTransport I/Os DNS messages (#760)

This diff refactors the DNSTransport model to receive in input a DNSQuery and return in output a DNSResponse.

The design of DNSQuery and DNSResponse takes into account the use case of a transport using getaddrinfo, meaning that we don't need to serialize and deserialize messages when using getaddrinfo.

The current codebase does not use a getaddrinfo transport, but I wrote one such a transport in the Websteps Winter 2021 prototype (https://github.com/bassosimone/websteps-illustrated/).

The design conversation that lead to producing this diff is https://github.com/ooni/probe/issues/2099

internal/model/mocks/dnsresponse_test.go

@@ -0,0 +1,105 @@
+package mocks
+
+import (
+	"bytes"
+	"errors"
+	"net"
+	"testing"
+
+	"github.com/miekg/dns"
+	"github.com/ooni/probe-cli/v3/internal/model"
+)
+
+func TestDNSResponse(t *testing.T) {
+	t.Run("Query", func(t *testing.T) {
+		qid := dns.Id()
+		query := &DNSQuery{
+			MockID: func() uint16 {
+				return qid
+			},
+		}
+		resp := &DNSResponse{
+			MockQuery: func() model.DNSQuery {
+				return query
+			},
+		}
+		out := resp.Query()
+		if out.ID() != query.ID() {
+			t.Fatal("invalid query")
+		}
+	})
+
+	t.Run("Bytes", func(t *testing.T) {
+		expected := []byte{0xde, 0xea, 0xad, 0xbe, 0xef}
+		resp := &DNSResponse{
+			MockBytes: func() []byte {
+				return expected
+			},
+		}
+		out := resp.Bytes()
+		if !bytes.Equal(expected, out) {
+			t.Fatal("invalid bytes")
+		}
+	})
+
+	t.Run("Rcode", func(t *testing.T) {
+		expected := dns.RcodeBadAlg
+		resp := &DNSResponse{
+			MockRcode: func() int {
+				return expected
+			},
+		}
+		out := resp.Rcode()
+		if out != expected {
+			t.Fatal("invalid rcode")
+		}
+	})
+
+	t.Run("DecodeLookupHost", func(t *testing.T) {
+		expected := errors.New("mocked error")
+		r := &DNSResponse{
+			MockDecodeLookupHost: func() ([]string, error) {
+				return nil, expected
+			},
+		}
+		out, err := r.DecodeLookupHost()
+		if !errors.Is(err, expected) {
+			t.Fatal("unexpected err", err)
+		}
+		if out != nil {
+			t.Fatal("unexpected out")
+		}
+	})
+
+	t.Run("DecodeHTTPS", func(t *testing.T) {
+		expected := errors.New("mocked error")
+		r := &DNSResponse{
+			MockDecodeHTTPS: func() (*model.HTTPSSvc, error) {
+				return nil, expected
+			},
+		}
+		out, err := r.DecodeHTTPS()
+		if !errors.Is(err, expected) {
+			t.Fatal("unexpected err", err)
+		}
+		if out != nil {
+			t.Fatal("unexpected out")
+		}
+	})
+
+	t.Run("DecodeNS", func(t *testing.T) {
+		expected := errors.New("mocked error")
+		r := &DNSResponse{
+			MockDecodeNS: func() ([]*net.NS, error) {
+				return nil, expected
+			},
+		}
+		out, err := r.DecodeNS()
+		if !errors.Is(err, expected) {
+			t.Fatal("unexpected err", err)
+		}
+		if out != nil {
+			t.Fatal("unexpected out")
+		}
+	})
+}