selftests/bpf: Add netkit tests for mac address and mtu

This adds two simple tests around setting MAC addresses in the different
netkit modes as well as syncing MTUs.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/prog_tests/tc_netkit.c

@@ -73,6 +73,25 @@ static int create_netkit(int mode, int policy, int peer_policy, int *ifindex,
 			 "up primary");
 	ASSERT_OK(system("ip addr add dev " netkit_name " 10.0.0.1/24"),
 			 "addr primary");
+
+	ASSERT_OK(system("ip link set dev " netkit_name " mtu 4000"),
+			 "set mtu");
+	ASSERT_OK(system("ip link show dev " netkit_peer " | grep 4000 > /dev/null"),
+			 "get mtu");
+	ASSERT_OK(system("ip link set dev " netkit_peer " mtu 1500"),
+			 "set mtu");
+	ASSERT_OK(system("ip link show dev " netkit_peer " | grep 4000 > /dev/null"),
+			 "get mtu");
+
+	if (mode == NETKIT_L3) {
+		ASSERT_EQ(system("ip link set dev " netkit_name
+				 " addr ee:ff:bb:cc:aa:dd 2> /dev/null"), 512,
+				 "set hwaddress");
+	} else {
+		ASSERT_OK(system("ip link set dev " netkit_name
+				 " addr ee:ff:bb:cc:aa:dd"),
+				 "set hwaddress");
+	}
 	if (same_netns) {
 		ASSERT_OK(system("ip link set dev " netkit_peer " up"),
 				 "up peer");