Also skip persistente tests related to UPDATE + ORDER BY for postgresql

PostgreSQL does not support updates with order by, and these tests are
failing randomly depending on the fixture loading order now.

Conflicts:

	activerecord/test/cases/associations/join_model_test.rb
	activerecord/test/cases/associations/nested_through_associations_test.rb
	activerecord/test/cases/clone_test.rb
	activerecord/test/cases/dup_test.rb
	activerecord/test/cases/relations_test.rb
	activerecord/test/cases/yaml_serialization_test.rb