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 <[email protected]>
Reviewed-by: Tino Reichardt <[email protected]>
Signed-off-by:	Alexander Motin <[email protected]>
Sponsored by:	iXsystems, Inc.
Closes: #17257

Author: amotin

tests/zfs-tests/tests/functional/slog/slog_006_pos.ksh

@@ -61,8 +61,7 @@ do
 			tdev=$(random_get $LDEV2)
 			log_must zpool replace $TESTPOOL $sdev $tdev
 			log_must display_status $TESTPOOL
-			# sleep 15 to make sure replacement completely.
-			log_must sleep 15
+			log_must zpool wait $TESTPOOL
 			log_must verify_slog_device \
 				$TESTPOOL $tdev 'ONLINE' $logtype