selftests: net: lib: add support for IPv6 mcast packet test

In order to test an IPv6 multicast packet we need to pass different tc
and mausezahn protocols only, so add a simple check for the destination
address which decides if we should generate an IPv4 or IPv6 mcast
packet.

Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>