Specify the interface through which packets should be transmitted so
that the test will pass regardless of the libnet version against which
mausezahn is linked.

Fixes: cab14d10 ("selftests: Add version of router_multipath.sh using nexthop objects")
Fixes: 3d578d87 ("selftests: forwarding: Test IPv4 weighted nexthops")
Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>