Disable DT_RUNPATH on NSS tests [BZ #28455]

The glibc internal NSS functions should always load NSS modules from
the system.  For testing purpose, disable DT_RUNPATH on NSS tests so
that the glibc internal NSS functions can load testing NSS modules
via DT_RPATH.

This partially fixes BZ #28455.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>