MDEV-34274: Test rpl.rpl_change_master_demote frequently fails on buildbot with "IO thread should not be running..."

The test rpl.rpl_change_master_demote used a `sleep 1` command
to give time for a START SLAVE UNTIL to start the slave threads
and wait for them to automatically die by UNTIL.  On machines
with heavy load (especially MSAN bb builders), one second was
not enough, and the test would fail due to the IO thread
still being up.

This patch fixes the test by replacing the sleep with specific
conditions to wait for. The test cannot wait for the IO or SQL
threads to start, as it would be possible that they would be
started and stopped by the time the MTR executor would check
the slave status. So instead, we test for proof that they
existed via the Connections status variable being incremented
by at least 2 (Connections just shows the global thread id).
At this point, we still can't use the wait_for_slave_to_stop
helper, as the SQL/IO_Running fields of SHOW SLAVE STATUS
may not be updated yet. So instead, we use
information_schema.processlist, which would show the presence
of the Slave_SQL/IO threads. So to "wait for the slave to stop",
we wait for the Slave_SQL/IO threads to be gone from the
processlist.