Fix loading fixtures in engine tests

fixture_path is a class attribute of ActiveSupport::TestCase, so
ActiveSupport::TestCase.method_defined?(:fixture_path=) would always
return false.

This should fix #4971

railties/lib/rails/generators/rails/plugin/templates/test/test_helper.rb

@@ -13,6 +13,6 @@
 Dir["#{File.dirname(__FILE__)}/support/**/*.rb"].each { |f| require f }
 
 # Load fixtures from the engine
-if ActiveSupport::TestCase.method_defined?(:fixture_path=)
+if ActiveSupport::TestCase.respond_to?(:fixture_path=)
   ActiveSupport::TestCase.fixture_path = File.expand_path("../fixtures", __FILE__)
 end