In an addition to test rpl.rpl_parallel_sbm added by MDEV-32265, the
test uses sleep statements alone to test Seconds_Behind_Master with
delayed replication. On slow running machines, the test can pass the
intended MASTER_DELAY duration and Seconds_Behind_Master can become
0, when the test expects the transaction to still be actively in a
delaying state.

This can be consistently reproduced by adding a sleep statement
before the call to

--let = query_get_value(SHOW SLAVE STATUS, Seconds_Behind_Master, 1)

to sleep past the delay end point.

This patch fixes this by locking the table which the delayed
transaction targets so Second_Behind_Master cannot be updated before
the test reads it for validation.