tests: copy /etc/pam.d/systemd-user from the host on Debian, Ubuntu

Fixes:
systemd-testsuite systemd[34]: PAM _pam_load_conf_file: unable to open /etc/pam.d/system-auth
systemd-testsuite systemd[34]: PAM _pam_load_conf_file: unable to open /etc/pam.d/system-auth
systemd-testsuite systemd[34]: user@0.service: Failed at step PAM spawning /lib/systemd/systemd: Operation not permitted
...
on Debian, Ubuntu