feat: dnsping using step-by-step (#831)

Reference issue for this pull request: https://github.com/ooni/probe/issues/2159

This diff refactors the `dnsping` experiment to use the [step-by-step measurement style](https://github.com/ooni/probe-cli/blob/master/docs/design/dd-003-step-by-step.md).

Co-authored-by: decfox <decfox@github.com>
Co-authored-by: Simone Basso <bassosimone@gmail.com>

internal/model/mocks/trace.go

@@ -15,6 +15,9 @@ import (
 type Trace struct {
 	MockTimeNow func() time.Time
 
+	MockOnDNSRoundTripForLookupHost func(started time.Time, reso model.Resolver, query model.DNSQuery,
+		response model.DNSResponse, addrs []string, err error, finished time.Time)
+
 	MockOnConnectDone func(
 		started time.Time, network, domain, remoteAddr string, err error, finished time.Time)
 
@@ -30,6 +33,11 @@ func (t *Trace) TimeNow() time.Time {
 	return t.MockTimeNow()
 }
 
+func (t *Trace) OnDNSRoundTripForLookupHost(started time.Time, reso model.Resolver, query model.DNSQuery,
+	response model.DNSResponse, addrs []string, err error, finished time.Time) {
+	t.MockOnDNSRoundTripForLookupHost(started, reso, query, response, addrs, err, finished)
+}
+
 func (t *Trace) OnConnectDone(
 	started time.Time, network, domain, remoteAddr string, err error, finished time.Time) {
 	t.MockOnConnectDone(started, network, domain, remoteAddr, err, finished)

internal/model/mocks/trace_test.go

@@ -4,6 +4,8 @@ import (
 	"crypto/tls"
 	"testing"
 	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
 )
 
 func TestTrace(t *testing.T) {
@@ -19,6 +21,28 @@ func TestTrace(t *testing.T) {
 		}
 	})
 
+	t.Run("OnDNSRoundTripForLookupHost", func(t *testing.T) {
+		var called bool
+		tx := &Trace{
+			MockOnDNSRoundTripForLookupHost: func(started time.Time, reso model.Resolver, query model.DNSQuery,
+				response model.DNSResponse, addrs []string, err error, finished time.Time) {
+				called = true
+			},
+		}
+		tx.OnDNSRoundTripForLookupHost(
+			time.Now(),
+			&Resolver{},
+			&DNSQuery{},
+			&DNSResponse{},
+			[]string{},
+			nil,
+			time.Now(),
+		)
+		if !called {
+			t.Fatal("not called")
+		}
+	})
+
 	t.Run("OnConnectDone", func(t *testing.T) {
 		var called bool
 		tx := &Trace{

internal/model/netx.go

@@ -303,6 +303,28 @@ type Trace interface {
 	// can use functionality exported by the ./internal/testingx pkg.
 	TimeNow() time.Time
 
+	// OnDNSRoundTripForLookupHost is used with a DNSTransport and called
+	// when the RoundTrip terminates.
+	//
+	// Arguments:
+	//
+	// - started is when we called transport.RoundTrip
+	//
+	// - reso is the parent resolver for the trace;
+	//
+	// - query is the non-nil DNS query we use for the RoundTrip
+	//
+	// - response is a valid DNS response, obtained after the RoundTrip;
+	//
+	// - addrs is the list of addresses obtained after the RoundTrip, which
+	// is empty if the RoundTrip failed
+	//
+	// - err is the result of DNSLookup; either an error or nil
+	//
+	// - finished is the time right after the RoundTrip
+	OnDNSRoundTripForLookupHost(started time.Time, reso Resolver, query DNSQuery,
+		response DNSResponse, addrs []string, err error, finished time.Time)
+
 	// OnConnectDone is called when connect terminates.
 	//
 	// Arguments: