[CLEAN] mail: tests: reduce a bit the boilerplate of tests to lessen

addons/mail/tests/common.py

addons/mail/tests/test_invite.py

addons/mail/tests/test_mail_channel.py

addons/mail/tests/test_mail_features.py

addons/mail/tests/test_mail_followers.py

addons/mail/tests/test_mail_gateway.py

addons/mail/tests/test_mail_message.py

addons/mail/tests/test_message_read.py

addons/portal/tests/test_portal.py

addons/portal_project/tests/test_access_rights.py

addons/project/tests/test_project_base.py