selftests: forwarding: lib: Add helpers for IP address handling

In order to generate IGMPv3 and MLDv2 packets on the fly, we will need
helpers to expand IPv4 and IPv6 addresses given as parameters in
mausezahn payload notation. Add helpers that do it.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>