[RFC 2/4] test: add test for af_packet

[Scott Mitchell]

Awesome! Some suggestions below but could easily be done in a followup
for incremental progress.

> +       /* TX burst */
> +       nb_tx = rte_eth_tx_burst(port_id, 0, bufs, BURST_SIZE);
> +
> +       /* Free any unsent mbufs */
> +       for (i = nb_tx; i < BURST_SIZE; i++)
> +               rte_pktmbuf_free(bufs[i]);
> +
> +       /* Small delay to allow stats update */
> +       rte_delay_us_block(1000);

Consider polling rte_eth_stats_get at a more frequent interval, break
when you see status update, and apply overall timeout/repeat limit ->
can make test more reliable and take less time when data is usually
available quickly

> +       /* Try to receive packets (non-blocking) */
> +       nb_rx = rte_eth_rx_burst(port_id, 0, bufs, BURST_SIZE);
> +
> +       /* Free any received mbufs */
> +       for (i = 0; i < nb_rx; i++)
> +               rte_pktmbuf_free(bufs[i]);
> +
> +       /* RX from tap interface without external traffic will return 0 */
> +       /* This test just verifies the RX path doesn't crash */

Consider adding a test that round trips data (tx -> rx) to exercise
the read loop.

> +       /* Get current MTU */
> +       ret = rte_eth_dev_get_mtu(port_id, &mtu);
> +       TEST_ASSERT(ret == 0, "Failed to get MTU");
> +
> +       /* Try to set a smaller MTU */
> +       ret = rte_eth_dev_set_mtu(port_id, 1400);

Consider getting the min/max supported MTU and setting it to verify it
works. My patch fixed some issues related to MTU and overhead of
frames that may have been caught by this.

> +
> +       /* Reset stats */
> +       ret = rte_eth_stats_reset(port_id);
> +       TEST_ASSERT(ret == 0, "Failed to reset stats");

Consider enhancing to introduce multiple threads (writer and reader)
to replicate

> +static int
> +test_af_packet_multi_queue(void)

Consider sharing rx/tx logic in common functions for single/multi-queue cases.