ZTS: deadman_sync fix

In the CI environment it's possible for events to be slightly
delayed resulting in 4, instead of 5, events appearing in the
log file. This isn't a problem and should be considered a
success to avoid false positive test results.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12625