The rpl.rpl_seconds_behind_master_spike test would sometimes
timeout or take a very long time to complete. This happened
because an MTR DEBUG_SYNC signal would be lost due to a
subsequent call to RESET. I.e., the slave SQL thread would
be paused due to the WAIT_FOR signal being lost, resulting in
either a failed test if the `select master_pos_wait` timeout
occurs first, or a very long run-time if the DBUG_SYNC timeout
occurs first.

The fix ensures that the MTR signal is processed by the slave
SQL thread before issuing the call to RESET

Reviewed By:
============
Andrei Elkin <andrei.elkin@mariadb.com>