When I open the Ghostscript test page on /mod/assign/feedback/editpdf/testgs.php, I get a fatal PHP error saying:

Fatal error: Class 'assign_feedback_plugin' not found in /var/www/html/moodle_test/mod/assign/feedback/editpdf/locallib.php on line 38

When I add the line

require_once($CFG->dirroot.'/mod/assign/feedbackplugin.php');

to the top of the /mod/assign/feedback/editpdf/locallib.php file, the test works and shows the smiley.

Interestingly:
1. The PDF editor works and loads although the Ghostscript test fails
2. On http://demo.moodle.net/mod/assign/feedback/editpdf/testgs.php, the test works although we have quite the same Moodle version running.