MDEV-12708 innodb.truncate_purge_debug fails sporadically in buildbot

The latest attempted fix for MDEV-11802 InnoDB purge fails to start when there is work to do is not complete. For now, work around the issue by making SHOW ENGINE INNODB STATUS call srv_wake_purge_thread_if_not_active() to wake up the purge.

Author: dr-m

storage/innobase/handler/ha_innodb.cc

@@ -16698,6 +16698,8 @@ innodb_show_status(
 DBUG_RETURN(0);
 }
 
+srv_wake_purge_thread_if_not_active();
+
 trx_t*	trx = check_trx_exists(thd);
 
 trx_assert_no_search_latch(trx);