selftests: forwarding: Add resilient multipath tunneling nexthop test

Add a resilient nexthop objects version of gre_multipath_nh.sh. Test
that both IPv4 and IPv6 overlays work with resilient nexthop groups
where the nexthops are two GRE tunnels.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>