Looking at the code, you should be able to set the `mailer_name` to make the anonymous class behave like it's parent. See:

* https://github.com/zuhao/rails/blob/refactor_actionmailer_tests/actionmailer/test/mailers/asset_mailer.rb#L2
* https://github.com/zuhao/rails/blob/refactor_actionmailer_tests/actionmailer/lib/action_mailer/base.rb#L476-L481
* https://github.com/zuhao/rails/blob/refactor_actionmailer_tests/actionmailer/lib/action_mailer/base.rb#L840