netinfotests: Do not assume the existence of /var/lib/vdsm/bonding-defaults.json

The cached bonding defaults file is not available before installation
so it cannot be used in 'make check'. Use the one from the source tree
in this case.

Change-Id: I8ec872739729e11ff9e4a9d30cdac36733d60bae
Signed-off-by: Ondřej Svoboda <osvoboda@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33766
Reviewed-by: Dan Kenigsberg <danken@redhat.com>