MDL-49998 unit tests: files left open are our fault.

Get rid of the gc_collect_cycles() that only was
hiding incorrectly left open files. This must be fixed
in code when possible. And pdf libs allow us to do so.

Note that a lot of gc_collect_cycles() were added
in MDL-47675, I'd recommend to take a look to all them,
being replaced by fixes in code when possible.

mod/assign/feedback/editpdf/tests/editpdf_test.php

@@ -52,13 +52,6 @@ class assignfeedback_editpdf_testcase extends mod_assign_base_testcase {
         parent::setUp();
     }
 
-    /**
-     * Tidy up open files that may be left open.
-     */
-    protected function tearDown() {
-        gc_collect_cycles();
-    }
-
     protected function create_assign_and_submit_pdf() {
         global $CFG;
         $assign = $this->create_instance(array('assignsubmission_onlinetext_enabled' => 1,