refactor: pass experiment arguments using a struct (#983)

Closes https://github.com/ooni/probe/issues/2358.

internal/engine/experiment/example/example.go

@@ -57,10 +57,10 @@ func (m Measurer) ExperimentVersion() string {
 var ErrFailure = errors.New("mocked error")
 
 // Run implements model.ExperimentMeasurer.Run.
-func (m Measurer) Run(
-	ctx context.Context, sess model.ExperimentSession,
-	measurement *model.Measurement, callbacks model.ExperimentCallbacks,
-) error {
+func (m Measurer) Run(ctx context.Context, args *model.ExperimentArgs) error {
+	callbacks := args.Callbacks
+	measurement := args.Measurement
+	sess := args.Session
 	var err error
 	if m.config.ReturnError {
 		err = ErrFailure

internal/engine/experiment/example/example_test.go

@@ -26,7 +26,12 @@ func TestSuccess(t *testing.T) {
 	sess := &mockable.Session{MockableLogger: log.Log}
 	callbacks := model.NewPrinterCallbacks(sess.Logger())
 	measurement := new(model.Measurement)
-	err := m.Run(ctx, sess, measurement, callbacks)
+	args := &model.ExperimentArgs{
+		Callbacks:   callbacks,
+		Measurement: measurement,
+		Session:     sess,
+	}
+	err := m.Run(ctx, args)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -47,7 +52,12 @@ func TestFailure(t *testing.T) {
 	ctx := context.Background()
 	sess := &mockable.Session{MockableLogger: log.Log}
 	callbacks := model.NewPrinterCallbacks(sess.Logger())
-	err := m.Run(ctx, sess, new(model.Measurement), callbacks)
+	args := &model.ExperimentArgs{
+		Callbacks:   callbacks,
+		Measurement: new(model.Measurement),
+		Session:     sess,
+	}
+	err := m.Run(ctx, args)
 	if !errors.Is(err, example.ErrFailure) {
 		t.Fatal("expected an error here")
 	}