Silence "setting dataset to sync always" message in ztest.

ztest outputs a message when testing sync=always no matter what the
verbosity level is. There is no point outputting this message for low
verbosity levels.

With this patch the message is only displayed at verbosity level 5 or
above. The result is less output pollution.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #951