The problem is that these tests run optimistic parallel replication with
non-transactional mysql.gtid_slave_pos table. Very occasionally InnoDB stats
update may race with one another and cause a parallel replication deadlock
kill after the mysql.gtid_slave_pos table has been updated. Since
mysql.gtid_slave_pos is non-transactional, the update cannot be rolled back,
and transaction retry will fail with a duplicate key error in
mysql.gtid_slave_pos.

Fixed by altering the storage engine to InnoDB for the table.
Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>