test: verify custom container interface

[troglobit]

Some customers are using container interfaces with custom interface names (inside the container) and with a custom MAC address based on the chassis mac, with an offset 06:00:00:00:00:00.

• Create two bridges (generic, non-VLAN filtering is fine), named br0 and br1
• Use two veth pairs veth0a<-->veth0b, and veth1a<-->veth1b
• Set custom container network interface names of veth0a and veth1a, use br0 and br1, respectively (yes, really)
• Set custom MAC address of veth0a (chassis + offset as above)
• Set veth0b and vet1b as bridge ports in br0 and br1, respectively
• Add IP addresses on all interfaces (except the bridge ports, of course) to verify connectivity
• SSH to the container to verify interface names

Add an ASCII drawing (or svg) to this test so to illustrate the connections. Something prettier than the below:

                                        container
                                       _____________
                                      |             |
                                      |   br1  br0  |
           br0              br1       |___/___/_____|
             \                \   vet1   /   /
              \                \________/   /
               \          veth0            /
                \_________________________/