nss: Do not mention NSS test modules in <gnu/lib-names.h>

They are not actually installed.  Use the nss_files version instead
in nss/Makefile, similar to how __nss_shlib_revision is derived
from LIBNSS_FILES_SO.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
(cherry picked from commit aefc79ab)