selftests: forwarding: Specify interface when invoking mausezahn

selftests: forwarding: Specify interface when invoking mausezahn

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: cab14d1087d9 ("selftests: Add version of router_multipath.sh using nexthop objects")
Fixes: 3d578d879517 ("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>