ZTS: Use swapctl to list swap devices on FreeBSD

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11503

tests/zfs-tests/tests/functional/zvol/zvol_swap/zvol_swap.cfg

@@ -35,6 +35,8 @@
 #
 if is_linux; then
 	SAVESWAPDEVS=$(swapon -s | nawk '(NR != 1) {print $1}')
+elif is_freebsd; then
+	SAVESWAPDEVS=$(swapctl -l | nawk '(NR != 1) {print $1}')
 else
 	SAVESWAPDEVS=$(swap -l | nawk '(NR != 1) {print $1}')
 fi