Add load hooks to all tests classes

Usually users extends tests classes doing something like:

    ActionView::TestCase.include MyCustomTestHelpers

This is bad because it will load the ActionView::TestCase right aways
and this will load ActionController::Base making its on_load hooks to
execute early than it should.

One way to fix this is using the on_load hooks of the components like:

    ActiveSupport.on_load(:action_view) do
      ActionView::TestCase.include MyCustomTestHelpers
    end

The problem with this approach is that the test extension will be only
load when ActionView::Base is loaded and this may happen too late in the
test.

To fix this we are adding hooks to people extend the test classes that
will be loaded exactly when the test classes are needed.

actionmailer/lib/action_mailer/test_case.rb

@@ -41,6 +41,7 @@ module ActionMailer
         setup :initialize_test_deliveries
         setup :set_expected_mail
         teardown :restore_test_deliveries
+        ActiveSupport.run_load_hooks(:action_mailer_test_case, self)
       end
 
       module ClassMethods

actionpack/lib/action_controller/test_case.rb

@@ -627,6 +627,7 @@ module ActionController
         include ActionDispatch::Assertions
         class_attribute :_controller_class
         setup :setup_controller_request_and_response
+        ActiveSupport.run_load_hooks(:action_view_test_case, self)
       end
 
       private

actionview/lib/action_view/test_case.rb

@@ -152,6 +152,7 @@ module ActionView
 
       included do
         setup :setup_with_controller
+        ActiveSupport.run_load_hooks(:action_view_test_case, self)
       end
 
     private

activejob/lib/active_job/test_case.rb

@@ -3,5 +3,7 @@ require "active_support/test_case"
 module ActiveJob
   class TestCase < ActiveSupport::TestCase
     include ActiveJob::TestHelper
+
+    ActiveSupport.run_load_hooks(:active_job_test_case, self)
   end
 end