Make ActionMailer::TestCase#read_fixture public

Since this method has been documented in the guides for ages, it's probably safe.

https://guides.rubyonrails.org/testing.html#revenge-of-the-fixtures

TODO: I don't like how these methods are documented on
ActionMailer::TestCase::Behavior, even though the API is meant to be
exposed through ActionMailer::TestCase

actionmailer/lib/action_mailer/test_case.rb

@@ -74,6 +74,15 @@ def determine_default_mailer(name)
         end
       end
 
+      # Reads the fixture file for the given mailer.
+      #
+      # This is useful when testing mailers by being able to write the body of
+      # an email inside a fixture. See the testing guide for a concrete example:
+      # https://guides.rubyonrails.org/testing.html#revenge-of-the-fixtures
+      def read_fixture(action)
+        IO.readlines(File.join(Rails.root, "test", "fixtures", self.class.mailer_class.name.underscore, action))
+      end
+
       private
         def initialize_test_deliveries
           set_delivery_method :test
@@ -110,10 +119,6 @@ def charset
         def encode(subject)
           Mail::Encodings.q_value_encode(subject, charset)
         end
-
-        def read_fixture(action)
-          IO.readlines(File.join(Rails.root, "test", "fixtures", self.class.mailer_class.name.underscore, action))
-        end
     end
 
     include Behavior