A minor stylistic fix and cleanup of mail deliveries.
Merge pull request #15085 from zuhao/refactor_actionmailer_log_subscriber_test

A minor stylistic fix and cleanup of mail deliveries.
Remove unnecessary setup and teardown.
docs, explain usage of `count` + `select`. Closes #15065. [ci skip]
Merge pull request #15086 from zuhao/refactor_actionmailer_mail_layout_test

Do not clear deliveries in setup.
Move state restoration from setup/teardown into affected test cases.
Merge pull request #15082 from zuhao/refactor_actionmailer_delivery_methods_test

Clean up deliveries after each test.
Use with_translation helper to clean up I18n stored translations.
Merge pull request #15083 from zuhao/refactor_actionmailer_i18n_with_controller_test

Use with_translation helper to clean up I18n stored translations.