selftests: drv-net: support generating iperf3 load

While we are not very interested in testing performance
it's useful to be able to generate a lot of traffic.
iperf is the simplest way of getting relatively high PPS.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240429144426.743476-6-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>