test: extend bridge-vlan-separation with MAC broadcast traffic

[troglobit]

The test ietf_interfaces/bridge_vlan_separation currently only use IP traffic, so a background load of broadcast (or multicast without IGMP/MLD snooping) traffic should be added.

Requirements:

• At least two VLANs
• At least two DUTs with a VLAN trunk between them
• Verify IP connectivity and isolation
• Verify MAC traffic isolation (see suggestion below)

My (@jovatn) suggestion for MAC traffic isolation would be to send some layer-2 broadcast and check that it reaches within the senders VLAN but not across to another VLAN.
Layer-2 broadcast could be achieved by pinging subnet broadcast (say "ping -b 10.0.0.255" on 10.0.0.0/24) or by ARP broadcast.