module ActionMailer::TestCase::Behavior

https://guides.rubyonrails.org/testing.html#revenge-of-the-fixtures
an email inside a fixture. See the testing guide for a concrete example:
This is useful when testing mailers by being able to write the body of

Reads the fixture file for the given mailer.

def read_fixture(action)
  IO.readlines(File.join(Rails.root, "test", "fixtures", self.class.mailer_class.name.underscore, action))
end