Add spec_system test for motd.tail

Since it's not possible to `exec login` to tell `pam_motd` to update the
real file, we have to call `run-parts` ourself.
Note about purge_directory breaking motd.tail

The concat of `motd.tail` is performed by `/etc/update-motd.d/99-footer`. If
the directory is purged and upstream defaults removed then this will stop
working as expected.