RQG testing: False alarms with symptom "mariabackup --backup" hangs

  Some session A executes BLOCK STAGE START with success.

  But there is no 100% guarantee this session will ever execute a BLOCK STAGE END.

  So some later started "mariabackup --backup" might fail to aquire the required backup lock over some long timespan.

The goal is to find some fix like

- simple, a small share of such false alarms will remain, reduced functional coverage

   Modify the RQG grammars conf/mariadb/table_stress_*.yy

- sophisticated, no reduced functional coverage

   Modify the RQG reporter 'Mariabackup_linux'.

   If the timeout gets exceeded search like

       SELECT * FROM information_schema.metadata_lock_info;

       THREAD_ID       LOCK_MODE       LOCK_DURATION   LOCK_TYPE       TABLE_SCHEMA    TABLE_NAME

       5       MDL_BACKUP_FLUSH        NULL    Backup lock

   and try to kill all connections holding a backup lock.

   In case that fixes the problem ignore the hang. Otherwise report a fail.