Problem:
========
The test logic checked for the wrong condition to validate that the
slave had caught up with the master. Specifically, it used the
thread stage of the IO and SQL thread to be in the “Waiting for
master to send event” and “Slave has read all relay log; waiting for
more updates” states, respectively. The problem exposed by this MDEV
is that, this state is also the initial slave state before reading
data from the primary (whereas the intended state was having already
read all available events from the primary and now waiting for new
events). This made the MTR test validate data that it had not yet
received, and thereby fail.

Solution:
========
Instead of using the IO/SQL thread states, use the existing helper
functions save_master_gtid.inc and sync_with_master_gtid.inc. Note
that the test result file also needed to be updated to reflect
this fix.

Special thanks to Angelique Sklavounos for pointing out that
--stop-position was not specified in any buildbot failures, as this
led to an IF block in the MTR test that was the source of the test
failure.

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