selftests: forwarding: Move IGMP- and MLD-related functions to lib

These functions will be helpful for other testsuites as well. Extract them
to a common place.

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