refactor(tracex): do not depend on strings for event names (#777)

Rather than matching a string, match a type.

This is more robust considering future refactorings.

We're confident the names did not change in _this_ refactoring
because we're still testing the same strings in the tests.

Part of https://github.com/ooni/probe/issues/2121

internal/engine/experiment/dash/dash.go

@@ -171,8 +171,8 @@ func (r runner) measure(
 		// of the latest connect time. We should have one sample in most
 		// cases, because the connection should be persistent.
 		for _, ev := range r.saver.Read() {
-			if ev.Name == netxlite.ConnectOperation {
-				connectTime = ev.Duration.Seconds()
+			if _, ok := ev.(*tracex.EventConnectOperation); ok {
+				connectTime = ev.Value().Duration.Seconds()
 			}
 		}
 		current.ConnectTime = connectTime

internal/engine/experiment/dash/dash_test.go

@@ -14,7 +14,6 @@ import (
 	"github.com/ooni/probe-cli/v3/internal/engine/mockable"
 	"github.com/ooni/probe-cli/v3/internal/engine/netx/tracex"
 	"github.com/ooni/probe-cli/v3/internal/model"
-	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 func TestRunnerLoopLocateFailure(t *testing.T) {
@@ -108,7 +107,7 @@ func TestRunnerLoopMeasureFailure(t *testing.T) {
 func TestRunnerLoopCollectFailure(t *testing.T) {
 	expected := errors.New("mocked error")
 	saver := new(tracex.Saver)
-	saver.Write(tracex.Event{Name: netxlite.ConnectOperation, Duration: 150 * time.Millisecond})
+	saver.Write(&tracex.EventConnectOperation{V: &tracex.EventValue{Duration: 150 * time.Millisecond}})
 	r := runner{
 		callbacks: model.NewPrinterCallbacks(log.Log),
 		httpClient: &http.Client{
@@ -152,7 +151,7 @@ func TestRunnerLoopCollectFailure(t *testing.T) {
 
 func TestRunnerLoopSuccess(t *testing.T) {
 	saver := new(tracex.Saver)
-	saver.Write(tracex.Event{Name: netxlite.ConnectOperation, Duration: 150 * time.Millisecond})
+	saver.Write(&tracex.EventConnectOperation{V: &tracex.EventValue{Duration: 150 * time.Millisecond}})
 	r := runner{
 		callbacks: model.NewPrinterCallbacks(log.Log),
 		httpClient: &http.Client{