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.

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);