selftests: add some benchmark for UDP GRO

Run on top of veth pair, using a dummy XDP program to enable the GRO.

 rfc v3 -> v1:
  - use ip route to attach the xdp helper to the veth

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>