Using Test Mail Collector to avoid sending emails to users during testing
After my attempt to use Devel module for  DevelMailLog also failed using the configuration setting

$config['system.mail']['interface']['default'] = 'devel_mail_log';


I concluded there must be something in my environment that is blocking these features.

After uninstalling the module "MailSystem", then either of the two approaches worked as intended while mail was still received when the configuration setting was omitted.