tests: fix pam_modules installation on Debian (and Debian-like systems)

libpam_modules installs modules into /lib/$(dpkg-architecture -qDEB_HOST_MULTIARCH)/security
on Debian

Fixes:
systemd-testsuite login[36]: PAM unable to dlopen(pam_group.so): /lib/security/pam_group.so: cannot open shared object file: No such file or directory
systemd-testsuite login[36]: PAM adding faulty module: pam_group.so
systemd-testsuite login[36]: PAM unable to dlopen(pam_limits.so): /lib/security/pam_limits.so: cannot open shared object file: No such file or directory
systemd-testsuite login[36]: PAM adding faulty module: pam_limits.so
...
etc