tests: net: iface: Fix interface context misuse

The iface test suite uses two different interface types with two
different context types and iid tests mixed up the two. It attempted
to create a mac address for Ethernet interface (which uses
struct eth_fake_context), however the net_iface_get_mac() function
only works with Dummy interface context (struct net_if_test). In
result the Ethernet interface context was corrupted (mac address
overwritten the promisc_mode flag).

Fix this by extracting mac generation code into a common function, and
use it in the Ethernet iface initialization phase instead of directly in
the test.

This was reported by UBSAN:
  tests/net/iface/src/main.c:239:34: runtime error: load of value 11,
  which is not a valid value for type '_Bool'

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>

tests/net/iface/src/main.c

@@ -78,20 +78,24 @@ struct net_if_test {
 	struct net_linkaddr ll_addr;
 };
 
-static uint8_t *net_iface_get_mac(const struct device *dev)
+static void test_create_mac(uint8_t *mac_buf)
 {
-	struct net_if_test *data = dev->data;
-
-	if (data->mac_addr[2] == 0x00) {
+	if (mac_buf[2] == 0x00) {
 		/* 00-00-5E-00-53-xx Documentation RFC 7042 */
-		data->mac_addr[0] = 0x00;
-		data->mac_addr[1] = 0x00;
-		data->mac_addr[2] = 0x5E;
-		data->mac_addr[3] = 0x00;
-		data->mac_addr[4] = 0x53;
-		data->mac_addr[5] = sys_rand8_get();
+		mac_buf[0] = 0x00;
+		mac_buf[1] = 0x00;
+		mac_buf[2] = 0x5E;
+		mac_buf[3] = 0x00;
+		mac_buf[4] = 0x53;
+		mac_buf[5] = sys_rand8_get();
 	}
+}
 
+static uint8_t *net_iface_get_mac(const struct device *dev)
+{
+	struct net_if_test *data = dev->data;
+
+	test_create_mac(data->mac_addr);
 	memcpy(data->ll_addr.addr, data->mac_addr, sizeof(data->mac_addr));
 	data->ll_addr.len = 6U;
 
@@ -207,6 +211,7 @@ static void eth_fake_iface_init(struct net_if *iface)
 
 	ctx->iface = iface;
 
+	test_create_mac(ctx->mac_address);
 	net_if_set_link_addr(iface, ctx->mac_address,
 			     sizeof(ctx->mac_address),
 			     NET_LINK_ETHERNET);
@@ -1364,8 +1369,6 @@ static void generate_iid(struct net_if *iface,
 	uint8_t *mac;
 	int ret;
 
-	(void)net_iface_get_mac(net_if_get_device(iface));
-
 	lladdr = net_if_get_link_addr(eth_iface);
 	mac = lladdr->addr;