The test that triggers multi-master conflict between two CTAS commands
uses LOCK/UNLOCK TABLES to block local CTAS from progress. It could
result in a race when UNLOCK TABLES command is issued a bit earlier
then needed, causing local CTAS to run further and change wsrep
transaction state, so that a different code path is taken later and
the original error gets overridden, causing the test to fail.
The solution is to replace LOCK/UNLOCK TABLES with debug sync points.
Signed-off-by: Julius Goryavsky <julius.goryavsky@mariadb.com>