selftests: forwarding: test for bridge port isolation

This test checks if the bridge port isolation feature works as expected
by performing ping/ping6 tests between hosts that are isolated (should
not work) and between an isolated and non-isolated hosts (should work).
Same test is performed for flooding from and to isolated and
non-isolated ports.

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