MDL-45605 Phpunit: Do not call gc_collect_cycles as part of reset.

This takes 25% of the time in running tests and there is no benefit. The
behaviour of destructors is the same before/after the patch.

lib/phpunit/classes/util.php

@@ -112,9 +112,6 @@ class phpunit_util extends testing_util {
         // Stop any message redirection.
         phpunit_util::stop_event_redirection();
 
-        // Release memory and indirectly call destroy() methods to release resource handles, etc.
-        gc_collect_cycles();
-
         // Show any unhandled debugging messages, the runbare() could already reset it.
         self::display_debugging_messages();
         self::reset_debugging();