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_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{