pam_zfs_key: tests: clean up the generated pam service config file

Remove the generated pam service config file
/etc/pam.d/pam_zfs_key_test on test cleanup, since the tests
shouldn't alter system state.

While here, move the pam service config file name into a variable.

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