tests: intsall /etc/login.defs

Fixes:
systemd-testsuite login[31]: cannot open login definitions /etc/login.defs [No such file or directory]
systemd-testsuite systemd[1]: Received SIGCHLD from PID 31 (login).
systemd-testsuite systemd[1]: Child 31 (login) died (code=exited, status=1/FAILURE)
systemd-testsuite systemd[1]: console-getty.service: Child 31 belongs to console-getty.service
systemd-testsuite systemd[1]: console-getty.service: Main process exited, code=exited, status=1/FAILURE
systemd-testsuite systemd[1]: console-getty.service: Changed running -> dead

on Debian/Ubuntu

Useful on other distros