Slow log entries causing main.subselect mtr failures.

In MDEV-7445, a test required an empty table as a data source. mysql.slow_log was chosen as this table, but some CI testers enable this in a worker as a command line option, meaning that this table may no longer be empty. If the table returns more than 1 row, the query fails, causing test failure.

This will clearly be a random failure, depending on OS workload.