Extend test coverage of Load and Render tabs to test the rendered content

After #2418897: Provide devel integration for all entity types automatically gets in, we need to add test coverage to the rendered content, as now we're just testing a '200' response in the url and not checking whether the content is correct or not.

Postponed on #2431263: Usage of print_r in _devel_print_object, ddebug_backtrace and drupal_debug causes out of memory errors and #2418897: Provide devel integration for all entity types automatically