This test was prone to failures for a few reasons, summarized below:

 1) MDEV-32168 introduced “only_running_threads=1” to
slave_stop.inc, which allowed the stop logic to bypass an
attempting-to-reconnect IO thread. That is, the IO thread could
realize the master shutdown in `read_event()`, and thereby call into
`try_to_reconnect()`. This would leave the IO thread up when the
test expected it to be stopped. Fixed by explicitly stopping the
IO thread and allowing an error state, as the above case would
lead to errno 2003.

 2) On slow systems (or those running profiling tools, e.g. MSAN),
the waiting-for-ack transaction can complete before the system
processes the `SHUTDOWN WAIT FOR ALL SLAVES`. There was shutdown
preparation logic in-between the transaction and shutdown itself,
which contributes to this problem. This patch also moves this
preparation logic before the transaction, so there is less to do
in-between the calls.

 3) Changed work-around for MDEV-28141 to use debug_sync instead
of sleep delay, as it was still possible to hit the bug on very
slow systems.

 4) Masked MTR variable reset with disable/enable query log

Reviewed By:
============
Kristian Nielsen <knielsen@knielsen-hq.org>