pam_zfs_key: tests: check if zfs load-key works on short passphrases

The pam_zfs_key pam module does not enforce a minimum password
length while changing the user password and thus the users home
dataset passphrase. To not end up with a dateset zfs load-key
can't load the key for, zfs load-key should not enforce a minimum
passphrase length. This adds a test for that.

Reviewed-by: Felix Dörre <felix@dogcraft.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Attila Fülöp <attila@fueloep.org>
Closes #12765
Closes #12651
Closes #12656