selftests: forwarding: lib: Add helpers to build IGMP/MLD leave packets

The testsuite that checks for mcast_max_groups functionality will need to
wipe the added groups as well. Add helpers to build an IGMP or MLD packets
announcing that host is leaving a given group.

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