Fix for #4971 - "can't use fixtures with a created engine"

A fix for loading the fixtures in the test_helper.rb generated for engines.

Changes the method of setting the fixture path; works around an issue checking if the fixture_path= method exists. As it is currently, ```ActiveSupport::TestCase.method_defined?(:fixture_path=)``` always returns false and the path will never be set.

In addition the fixtures were not being loaded.

References issue #4971