selftests: bridge_fdb_local_vlan_0: Test FDB vs. NET_ADDR_SET behavior

The previous patch fixed an issue whereby no FDB entry would be created for
the bridge itself on VLAN 0 under some circumstances. This could break
forwarding. Add a test for the fix.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/137cc25396f5a4f407267af895a14bc45552ba5f.1758550408.git.petrm@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>