The test's header is not written to follow strictly a correct order
of checks by mtr at test start which may lead to an error. E.g

./mtr --mysqld=--binlog-format=row rpl.rpl_using_gtid_default

to
At line 175: query 'SET GLOBAL gtid_slave_pos= ""' failed: ER_SLAVE_MUST_STOP (1198): This operation cannot be performed as you have a running slave ''; run STOP SLAVE '' first

Fixed to require the binlog format first in the test header.