selftests: net: add macvlan multicast test for shared source MAC

Add a selftest that verifies multicast delivery to a macvlan bridge
port when the source MAC of the incoming frame matches the macvlan's
own MAC address.

This scenario occurs with protocols like VRRP where multiple hosts
share the same virtual MAC address. Without the corresponding kernel
change, macvlan bridge mode does not handle this case and the
multicast frame is not delivered.

Signed-off-by: Kibaek Yoo <psykibaek@gmail.com>
Link: https://patch.msgid.link/20260228071613.4360-2-psykibaek@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>