The failure happens due to a race condition between processing
a row event (INSERT) and an automatically generated event
DROP TEMPORARY TABLE. Even though DROP has a higher GTID, it can
become visible in @@gtid_slave_pos before the row event with
a lower GTID has been committed. Since the test makes the slave
to synchronize with the master using GTID, the waiting stops
as soon as GTID of the DROP TEMPORARY TABLE becomes visible,
and if changes from the previous event haven't been applied yet,
the error occurs.

According to Kristian (see the comment to MDEV-10631), the real
problem is that DROP TEMPORARY TABLE is logged in the row mode
at all. For this particular test, since DROP does not do anything,
nothing prevents it from competing with the prior transaction.

The workaround for the test is to add a meaningful event
after DROP TEMPORARY TABLE, so that the slave would wait on its
GTID instead of the one from DROP.

Additionally (unrelated to this problem) removed FLUSH TABLES,
which, as the comment stated, should have been removed after
MDEV-6403 was fixed.