feat(ooniprobe): add torsf to experimental group (#670)

Reference issue: https://github.com/ooni/probe/issues/1917.

I needed to change the summary key type returned by `torsf` to be a value. It seems the DB layer assumes that. If we pass it a pointer, it panics because it's experiment a value rather than a pointer 🤷.

internal/engine/experiment/torsf/torsf.go

@@ -173,5 +173,5 @@ type SummaryKeys struct {
 
 // GetSummaryKeys implements model.ExperimentMeasurer.GetSummaryKeys.
 func (m *Measurer) GetSummaryKeys(measurement *model.Measurement) (interface{}, error) {
-	return &SummaryKeys{IsAnomaly: false}, nil
+	return SummaryKeys{IsAnomaly: false}, nil
 }

internal/engine/experiment/torsf/torsf_test.go

@@ -160,7 +160,7 @@ func TestGetSummaryKeys(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	rsk := sk.(*SummaryKeys)
+	rsk := sk.(SummaryKeys)
 	if rsk.IsAnomaly {
 		t.Fatal("expected no anomaly here")
 	}