Include the selector assertions on the test case

We don't need to require users to include this module on
ActionMailer::TestCase

actionmailer/lib/action_mailer/test_case.rb

@@ -1,4 +1,5 @@
 require 'active_support/test_case'
+require 'rails-dom-testing'
 
 module ActionMailer
   class NonInferrableMailerError < ::StandardError
@@ -15,6 +16,7 @@ module ActionMailer
 
       include ActiveSupport::Testing::ConstantLookup
       include TestHelper
+      include Rails::Dom::Testing::Assertions::SelectorAssertions
 
       included do
         class_attribute :_mailer_class

actionmailer/test/assert_select_email_test.rb

@@ -1,9 +1,6 @@
 require 'abstract_unit'
-require 'rails-dom-testing'
 
 class AssertSelectEmailTest < ActionMailer::TestCase
-  include Rails::Dom::Testing::Assertions::SelectorAssertions
-
   class AssertSelectMailer < ActionMailer::Base
     def test(html)
       mail body: html, content_type: "text/html",

actionview/test/template/atom_feed_helper_test.rb

@@ -207,8 +207,6 @@ end
 class AtomFeedTest < ActionController::TestCase
   tests ScrollsController
 
-  include Rails::Dom::Testing::Assertions::SelectorAssertions
-
   def setup
     super
     @request.host = "www.nextangle.com"