ZTS: Remove fixed sleeps from slog_006_pos

Replace sleep 15 with zpool wait, which should take much less
than the 15 seconds. And considering it is called 16 times, this
should save us up to 4 minutes total.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes: #17257