fix: ensure experiments return nil when we want to submit (#654)

Since https://github.com/ooni/probe-cli/pull/527, if an experiment
returns an error, the corresponding measurement is not submitted since
the semantics of returning an error is that something fundamental
went wrong (e.g., we could not parse the input URL).

This diff ensures that all experiments only return and error when
something fundamental was wrong and return nil otherwise.

Reference issue: https://github.com/ooni/probe/issues/1808.

internal/engine/experiment/run/dnscheck.go

@@ -2,7 +2,6 @@ package run
 
 import (
 	"context"
-	"sync"
 
 	"github.com/ooni/probe-cli/v3/internal/engine/experiment/dnscheck"
 	"github.com/ooni/probe-cli/v3/internal/model"
@@ -10,7 +9,6 @@ import (
 
 type dnsCheckMain struct {
 	Endpoints *dnscheck.Endpoints
-	mu        sync.Mutex
 }
 
 func (m *dnsCheckMain) do(ctx context.Context, input StructuredInput,

internal/engine/experiment/run/doc.go

@@ -0,0 +1,4 @@
+// Package run contains code to run other experiments.
+//
+// This code is currently alpha.
+package run

internal/engine/experiment/run/run.go

@@ -1,6 +1,3 @@
-// Package run contains code to run other experiments.
-//
-// This code is currently alpha.
 package run
 
 import (

internal/engine/experiment/run/run_test.go

@@ -63,7 +63,7 @@ func TestRunURLGetterWithCancelledContext(t *testing.T) {
 	sess := &mockable.Session{MockableLogger: log.Log}
 	callbacks := model.NewPrinterCallbacks(log.Log)
 	err := measurer.Run(ctx, sess, measurement, callbacks)
-	if err == nil {
+	if err != nil { // here we expected nil b/c we want to submit the measurement
 		t.Fatal(err)
 	}
 	if len(measurement.Extensions) != 6 {