This commit contains only a mtr test for reproducing the issue in MDEV-29512
The actual fix will be pushed in wsrep-lib repository

The hanging in MDEV-29512 happens when binlog purging is attempted, and there is
one local BF aborted transaction waiting for commit monitor.

The test will launch two node cluster and enable binlogging with expire log days,
to force binlog purging to happen.
A local transaction is executed so that will become BF abort victim, and has advanced
to replication stage waiting for commit monitor for final cleanup (to mark position in innodb)
after that, applier is released to complete the BF abort and due to binlog configuration,
starting the binlog purging. This is where the hanging would occur, if code is buggy
Reviewed-by: Jan Lindström <jan.lindstrom@mariadb.com>