selftests: forwarding: Add device-only nexthop test

In a similar fashion to router_multipath.sh and its nexthop objects
version router_mpath_nh.sh, create a nexthop objects version of
router.sh.

It reuses the same topology, but uses device-only nexthop objects
instead of legacy ones.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>