Add more ostruct requires

`ostruct` was being implictly required by the `json` gem. But once it
was upgraded, these tests failed to initialize `OpenStruct`.

While we're trying to remove `ostruct` usage in rails#51510, CI is currently
failing so I'm pushing these in the mean time.

Author: eileencodes

actionmailer/test/url_test.rb

@@ -2,6 +2,7 @@
 
 require "abstract_unit"
 require "action_controller"
+require "ostruct"
 
 class WelcomeController < ActionController::Base
 end

actionview/test/template/controller_helper_test.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "abstract_unit"
+require "ostruct"
 
 class ControllerHelperTest < ActionView::TestCase
   tests ActionView::Helpers::ControllerHelper

actionview/test/template/form_helper_test.rb

@@ -2,6 +2,7 @@
 
 require "abstract_unit"
 require "controller/fake_models"
+require "ostruct"
 
 class FormHelperTest < ActionView::TestCase
   include RenderERBUtils

actionview/test/template/record_identifier_test.rb

@@ -2,6 +2,7 @@
 
 require "abstract_unit"
 require "controller/fake_models"
+require "ostruct"
 
 class RecordIdentifierTest < ActiveSupport::TestCase
   include ActionView::RecordIdentifier

railties/test/application/rake/dbs_test.rb

@@ -2,6 +2,7 @@
 
 require "isolation/abstract_unit"
 require "env_helpers"
+require "ostruct"
 
 module ApplicationTests
   module RakeTests