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

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

internal/engine/experiment/signal/signal.go

@@ -141,8 +141,10 @@ func (m Measurer) ExperimentVersion() string {
 }
 
 // Run implements 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
 	ctx, cancel := context.WithTimeout(ctx, 60*time.Second)
 	defer cancel()
 	urlgetter.RegisterExtensions(measurement)

internal/engine/experiment/signal/signal_test.go

@@ -25,14 +25,14 @@ func TestNewExperimentMeasurer(t *testing.T) {
 func TestGood(t *testing.T) {
 	measurer := signal.NewExperimentMeasurer(signal.Config{})
 	measurement := new(model.Measurement)
-	err := measurer.Run(
-		context.Background(),
-		&mockable.Session{
+	args := &model.ExperimentArgs{
+		Callbacks:   model.NewPrinterCallbacks(log.Log),
+		Measurement: measurement,
+		Session: &mockable.Session{
 			MockableLogger: log.Log,
 		},
-		measurement,
-		model.NewPrinterCallbacks(log.Log),
-	)
+	}
+	err := measurer.Run(context.Background(), args)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -103,14 +103,14 @@ func TestBadSignalCA(t *testing.T) {
 		SignalCA: "INVALIDCA",
 	})
 	measurement := new(model.Measurement)
-	err := measurer.Run(
-		context.Background(),
-		&mockable.Session{
+	args := &model.ExperimentArgs{
+		Callbacks:   model.NewPrinterCallbacks(log.Log),
+		Measurement: measurement,
+		Session: &mockable.Session{
 			MockableLogger: log.Log,
 		},
-		measurement,
-		model.NewPrinterCallbacks(log.Log),
-	)
+	}
+	err := measurer.Run(context.Background(), args)
 	if err.Error() != "AppendCertsFromPEM failed" {
 		t.Fatal("not the error we expected")
 	}