selftests: forwarding: test for bridge mcast traffic after report and leave

This test is split in two, the first part checks if a report creates a
corresponding mdb entry and if traffic is properly forwarded to it, and
the second part checks if the mdb entry is deleted after a leave and
if traffic is *not* forwarded to it. Since the mcast querier is enabled
we should see standard mcast snooping bridge behaviour.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>