selftest: tun: Introduce tuntap_helpers.h header for TUN/TAP testing

selftest: tun: Introduce tuntap_helpers.h header for TUN/TAP testing

Introduce rtnetlink manipulation and packet construction helpers that
will simplify the later creation of more related test cases. This avoids
duplicating logic across different test cases.

This new header will contain:
 - YNL-based netlink management utilities.
 - Helpers for ip link, ip address, ip neighbor and ip route operations.
 - Packet construction and manipulation helpers.

Signed-off-by: Xu Du <xudu@redhat.com>
Link: https://patch.msgid.link/91f905715c69c75f7bf72d43388921fde6c34989.1768979440.git.xudu@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>