MDEV-34274 did not fix the test failure. The test has a START SLAVE
UNTIL condition, where we can't use sync_with_master_gtid.inc,
wait_for_slave_to_start.inc, or wait_for_slave_to_stop.inc because
our MTR connection thread races with the start/stop of the SQL/IO
threads. So instead, for slave start, we prove the threads started
by waiting for the connection count to increase by 2; and for slave
stop, we wait for the processlist count to return to its pre start
slave number.