selftest: add selftest for anycast notifications

This commit adds a new kernel selftest to verify RTNLGRP_IPV6_ACADDR
notifications. The test works by adding/removing a dummy interface,
enabling packet forwarding, and then confirming that user space can
correctly receive anycast notifications.

The test relies on the iproute2 version to be 6.13+.

Tested by the following command:
$ vng -v --user root --cpus 16 -- \
make -C tools/testing/selftests TARGETS=net
TEST_PROGS=rtnetlink_notification.sh \
TEST_GEN_PROGS="" run_tests

Cc: Maciej Żenczykowski <maze@google.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Yuyang Huang <yuyanghuang@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>