fix(dnscheck): log "ok" rather than "<nil>" on success (#695)

See https://github.com/ooni/probe/issues/2020

internal/model/logger.go

@@ -61,3 +61,11 @@ func (logDiscarder) Warn(msg string) {}
 
 // Warnf implements Logger.Warnf
 func (logDiscarder) Warnf(format string, v ...interface{}) {}
+
+// ErrorToStringOrOK emits "ok" on "<nil>"" values for success.
+func ErrorToStringOrOK(err error) string {
+	if err != nil {
+		return err.Error()
+	}
+	return "ok"
+}

internal/model/logger_test.go

@@ -1,6 +1,9 @@
 package model
 
-import "testing"
+import (
+	"io"
+	"testing"
+)
 
 func TestDiscardLoggerWorksAsIntended(t *testing.T) {
 	logger := DiscardLogger
@@ -11,3 +14,20 @@ func TestDiscardLoggerWorksAsIntended(t *testing.T) {
 	logger.Warn("foo")
 	logger.Warnf("%s", "foo")
 }
+
+func TestErrorToStringOrOK(t *testing.T) {
+	t.Run("on success", func(t *testing.T) {
+		expectedResult := ErrorToStringOrOK(nil)
+		if expectedResult != "ok" {
+			t.Fatal("expected ok")
+		}
+	})
+
+	t.Run("on failure", func(t *testing.T) {
+		err := io.EOF
+		expectedResult := ErrorToStringOrOK(err)
+		if expectedResult != err.Error() {
+			t.Fatal("not the result we expected", expectedResult)
+		}
+	})
+}