selftests: forwarding: Add a tunnel-based test for L3 HW stats

Add a selftest that uses an IPIP topology and tests that L3 HW stats
reflect the traffic in the tunnel.

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