ZTS: Update deadman_sync threshold

Lower the minimum number of expected deadman events from 4 to 3. All
that is strictly required is a single event to consider the test a
pass. However, since I've never seen a count of less than 3 reported
by the CI that should be sufficient.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #16575