Testing failing on 8.x Branch

There was a change on the 23 may #2474959: CAPTCHA session reuse attack detected.

It changed code in _captcha_get_posted_captcha_info to "Invalidate CAPTCHA token to avoid reuse", adding execute to the database update call. This has resulted in the testPersistenceAlways test in CaptchaPersistenceTestCase to fail.

        // Invalidate CAPTCHA token to avoid reuse.
        \Drupal::database()->update('captcha_sessions')
          ->fields(['token' => NULL])
          ->condition('csid', $posted_captcha_sid)
          ->execute();