Fix failing AMo isolated tests

activemodel/test/cases/serializeration/json_serialization_test.rb

 require 'cases/helper'
 require 'models/contact'
+require 'active_support/core_ext/object/instance_variables'
 
 class Contact
+  extend ActiveModel::Naming
   include ActiveModel::Serializers::JSON
 
   def attributes

activemodel/test/cases/serializeration/xml_serialization_test.rb

 require 'cases/helper'
 require 'models/contact'
+require 'active_support/core_ext/object/instance_variables'
 
 class Contact
+  extend ActiveModel::Naming
   include ActiveModel::Serializers::Xml
 
   def attributes

activemodel/test/cases/validations/presence_validation_test.rb

@@ -5,6 +5,7 @@
 require 'models/topic'
 require 'models/developer'
 require 'models/person'
+require 'models/custom_reader'
 
 class PresenceValidationTest < ActiveModel::TestCase
   include ActiveModel::TestsDatabase

activemodel/test/cases/validations/with_validation_test.rb

 # encoding: utf-8
 require 'cases/helper'
+require 'cases/tests_database'
 
 require 'models/topic'
 
 class ValidatesWithTest < ActiveRecord::TestCase
+  include ActiveModel::TestsDatabase
   include ActiveModel::ValidationsRepairHelper
 
   repair_validations(Topic)