Ensure that iperf tests run across their designated interfaces

Code in this branch is designed to ensure that iperf tests run traffic across the correct interfaces.

There are two main goals:
- Enforce (using ARP rules/IP routes) that traffic is directed out of the correct interface, and reaches the correct interface on the other side
- Check the number of bytes that flow through a given interface against the expected value in order to ensure that we are testing what we think we're testing.