selftests: drv-net: add ability to wait for at least N packets to load gen

Teach the load generator how to wait for at least given number
of packets to be received. This will be useful for filtering
where we'll want to send a non-trivial number of packets and
make sure they landed in right queues.

Reviewed-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20240626012456.2326192-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>