MDL-38212 fix related unit tests.

For statslib tests was enough to require cronlib.php as
far as all the tests there are using ob-capture / output
expectations, so the new function does not bork anything.

For pluginlib tests finally the use was deleted because
it's a part of cron not interesting (should be always "cheap")
and to keep it there we should be adding a bunch of output
expectations under some cases, for practically nothing.

lib/pluginlib.php

@@ -809,7 +809,6 @@ class available_update_checker {
         $start = mktime(1, 0, 0, date('n', $now), date('j', $now), date('Y', $now)); // 01:00 AM today local time
         if ($now > $start + $offset) {
             $this->cron_mtrace('Regular daily check for available updates ... ', '');
-            cron_trace_time_and_memory();
             $this->cron_execute();
             return;
         }