selftests: drv-net: rss_ctx: factor out send traffic and check

Wrap up sending traffic and checking in which queues it landed
in a helper.

The method used for testing is to send a lot of iperf traffic
and check which queues received the most packets. Those should
be the queues where we expect iperf to land - either because we
installed a filter for the port iperf uses, or we didn't and
expect it to use context 0.

Contexts get disjoint queue sets, but the main context (AKA context 0)
may receive some background traffic (noise).

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20240708213627.226025-3-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>