dracut/mount-zfs.sh: quote expansion on zpool test

Bring over some of the improvements from dracut/zfs-load-key.sh,
shellcheck is slightly quieter as well

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11198