galera suite MTR tests MW-328B and MW-328C sporadic failures

MariaDB versions 10.1 and 10.2 show sporadic failures with mtr galera suite tests MW- 328B and 328C. Failure is about excessive warnings found in error log after test execution, like here:

galera.MW-328B 'xtradb'                  [ fail ]  Found warnings/errors in server log file!

        Test ended at 2018-04-24 12:27:59

line

safe_mutex: Found wrong usage of mutex 'LOCK_wsrep_thd' and 'LOCK_thd_data'

^ Found warnings in /home/seppo/work/wsrep/mariadb-server/mysql-test/var/log/mysqld.2.err

In 10.3, similar warnings happen, but the deadlocking mutexes are: LOCK_thd_kill and LOCK_wsrep_thd

How to Reproduce:
1. Prepare debug build of 10.1, 10.2 or 10.3 version
2. start mtr test, e.g. like here:
./mtr galera.MW-328B --repeat=100