Override CiviCRM settings for test and local environments

Unlike in Drupal itself, where the settings.php file is meant to be extended by local.settings.php etc., CiviCRM’s civicrm.settings.php does not encourage such behavior— it even has define statements, which do not react well to being duplicated.

The use case of https://docs.civicrm.org/sysadmin/en/latest/customize/settings/