test: add AVB protocol test suite with loopback transport

Add a test suite for the AVB (Audio Video Bridging) protocol stack that
runs entirely in software, requiring no hardware, root privileges, or
running PipeWire daemon.

The loopback transport (avb-transport-loopback.h) replaces raw AF_PACKET
sockets with in-memory packet capture, using a synthetic MAC address and
eventfd for protocol handlers that need a valid fd.

Test utilities (test-avb-utils.h) provide helpers for creating test
servers, injecting packets, advancing time, and building ADP packets.

Tests cover:
- ADP entity available/departing/discover/timeout
- MRP attribute lifecycle (create, begin, join)
- Milan v1.2 mode server creation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/modules/module-avb/avb-transport-loopback.h

@@ -0,0 +1,149 @@
+/* AVB support */
+/* SPDX-FileCopyrightText: Copyright © 2026 PipeWire contributors */
+/* SPDX-License-Identifier: MIT */
+
+#ifndef AVB_TRANSPORT_LOOPBACK_H
+#define AVB_TRANSPORT_LOOPBACK_H
+
+#include <string.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <sys/eventfd.h>
+
+#include "internal.h"
+#include "packets.h"
+
+#define AVB_LOOPBACK_MAX_PACKETS	64
+#define AVB_LOOPBACK_MAX_PACKET_SIZE	2048
+
+struct avb_loopback_packet {
+	uint8_t dest[6];
+	uint16_t type;
+	size_t size;
+	uint8_t data[AVB_LOOPBACK_MAX_PACKET_SIZE];
+};
+
+struct avb_loopback_transport {
+	struct avb_loopback_packet packets[AVB_LOOPBACK_MAX_PACKETS];
+	int packet_count;
+	int packet_read;
+};
+
+static inline int avb_loopback_setup(struct server *server)
+{
+	struct avb_loopback_transport *t;
+	static const uint8_t test_mac[6] = { 0x02, 0x00, 0x00, 0x00, 0x00, 0x01 };
+
+	t = calloc(1, sizeof(*t));
+	if (t == NULL)
+		return -errno;
+
+	server->transport_data = t;
+
+	memcpy(server->mac_addr, test_mac, 6);
+	server->ifindex = 1;
+	server->entity_id = (uint64_t)server->mac_addr[0] << 56 |
+			(uint64_t)server->mac_addr[1] << 48 |
+			(uint64_t)server->mac_addr[2] << 40 |
+			(uint64_t)0xff << 32 |
+			(uint64_t)0xfe << 24 |
+			(uint64_t)server->mac_addr[3] << 16 |
+			(uint64_t)server->mac_addr[4] << 8 |
+			(uint64_t)server->mac_addr[5];
+
+	return 0;
+}
+
+static inline int avb_loopback_send_packet(struct server *server,
+		const uint8_t dest[6], uint16_t type, void *data, size_t size)
+{
+	struct avb_loopback_transport *t = server->transport_data;
+	struct avb_loopback_packet *pkt;
+	struct avb_ethernet_header *hdr = (struct avb_ethernet_header*)data;
+
+	if (t->packet_count >= AVB_LOOPBACK_MAX_PACKETS)
+		return -ENOSPC;
+	if (size > AVB_LOOPBACK_MAX_PACKET_SIZE)
+		return -EMSGSIZE;
+
+	/* Fill in the ethernet header like the raw transport does */
+	memcpy(hdr->dest, dest, 6);
+	memcpy(hdr->src, server->mac_addr, 6);
+	hdr->type = htons(type);
+
+	pkt = &t->packets[t->packet_count % AVB_LOOPBACK_MAX_PACKETS];
+	memcpy(pkt->dest, dest, 6);
+	pkt->type = type;
+	pkt->size = size;
+	memcpy(pkt->data, data, size);
+	t->packet_count++;
+
+	return 0;
+}
+
+/**
+ * Return a dummy fd for protocol handlers that create their own sockets.
+ * Uses eventfd so pw_loop_add_io() has a valid fd to work with.
+ */
+static inline int avb_loopback_make_socket(struct server *server,
+		uint16_t type, const uint8_t mac[6])
+{
+	int fd;
+
+	fd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);
+	if (fd < 0)
+		return -errno;
+
+	return fd;
+}
+
+static inline void avb_loopback_destroy(struct server *server)
+{
+	free(server->transport_data);
+	server->transport_data = NULL;
+}
+
+static const struct avb_transport_ops avb_transport_loopback = {
+	.setup = avb_loopback_setup,
+	.send_packet = avb_loopback_send_packet,
+	.make_socket = avb_loopback_make_socket,
+	.destroy = avb_loopback_destroy,
+};
+
+/** Get the number of captured sent packets */
+static inline int avb_loopback_get_packet_count(struct server *server)
+{
+	struct avb_loopback_transport *t = server->transport_data;
+	return t->packet_count - t->packet_read;
+}
+
+/** Read the next captured sent packet, returns packet size or -1 */
+static inline int avb_loopback_get_packet(struct server *server,
+		void *buf, size_t bufsize)
+{
+	struct avb_loopback_transport *t = server->transport_data;
+	struct avb_loopback_packet *pkt;
+
+	if (t->packet_read >= t->packet_count)
+		return -1;
+
+	pkt = &t->packets[t->packet_read % AVB_LOOPBACK_MAX_PACKETS];
+	t->packet_read++;
+
+	if (pkt->size > bufsize)
+		return -1;
+
+	memcpy(buf, pkt->data, pkt->size);
+	return pkt->size;
+}
+
+/** Clear all captured packets */
+static inline void avb_loopback_clear_packets(struct server *server)
+{
+	struct avb_loopback_transport *t = server->transport_data;
+	t->packet_count = 0;
+	t->packet_read = 0;
+}
+
+#endif /* AVB_TRANSPORT_LOOPBACK_H */