selftests: net: bridge: igmp: add IGMPv3 entries' state helpers

Add helpers which will be used in subsequent tests, they are:
 - check_sg_entries: check for proper source list and S,G entry
   existence
 - check_sg_fwding: check for proper traffic forwarding/blocking
 - check_sg_state: check for proper blocked/forwarding entry state

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