Calls to captcha_set_form_id_setting() using $form_id should really use constants.

At the moment any change to a form_id in drupal means the tests have to be changed. Centralizing these values would stop this from happening in the future.

Could possibly implement this along with #2024901: Move to file-based configuration system

Changing to configuration system would mean you would call something along the lines of config->fieldvalue where field value could be a constant from the config.