Make test:isolated run without bundler for Action Mailer

Action Mailer tests weren't able to run in isolation without the bundle
exec prefix since we were requiring gems before requiring abstract_unit.

We don't need the `gem` call thus and the require_relative since the
test directory should be present in the load path when we run any test.

actionmailer/test/message_delivery_test.rb

 # encoding: utf-8
-gem 'activejob'
-require 'active_job'
 require 'abstract_unit'
+require 'active_job'
 require 'minitest/mock'
-require_relative 'mailers/delayed_mailer'
+require 'mailers/delayed_mailer'
 
 class MessageDeliveryTest < ActiveSupport::TestCase