Fix a state leak in `autosave_association_test`

The test added in 85465ed3e6 was passing
when the file was run on its own, but failing when the entire suite was
run since this test modifies the class and doesn't clean up.

activerecord/test/cases/autosave_association_test.rb

@@ -1044,11 +1044,16 @@ class TestAutosaveAssociationOnAHasOneAssociation < ActiveRecord::TestCase
   end
 
   def test_should_not_ignore_different_error_messages_on_the_same_attribute
+    old_validators = Ship._validators.deep_dup
+    old_callbacks = Ship._validate_callbacks.deep_dup
     Ship.validates_format_of :name, :with => /\w/
     @pirate.ship.name   = ""
     @pirate.catchphrase = nil
     assert @pirate.invalid?
     assert_equal ["can't be blank", "is invalid"], @pirate.errors[:"ship.name"]
+  ensure
+    Ship._validators = old_validators if old_validators
+    Ship._validate_callbacks = old_callbacks if old_callbacks
   end
 
   def test_should_still_allow_to_bypass_validations_on_the_associated_model