selftests: net: bridge: add test for mldv2 *,g auto-add

When we have *,G ports in exclude mode and a new S,G,port is added
the kernel has to automatically create an S,G entry for each exclude
port to get proper forwarding.

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