MDEV-22125 : galera.galera_drop_multi MTR failed: InnoDB: MySQL is trying to drop database `fts`.`` though there are still open handles

MDEV-22140 galera.galera_drop_database MTR failed: InnoDB: MySQL is trying to drop database `fts`.`` though there are still open handles

Add wait conditions to wait that all operations are done in both
nodes.