pam_zfs_key: change test user name to conform to standards
The useradd(8) command on my system won't accept login names with
uppercase letters in them, so adjust 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 #12665