Expose MITM proxy dumps

[kegsay]

We currently use mitmproxy everywhere but we aren't making use of its features fully. It would be nice if:

• we can capture all traffic in a test and save the resulting dump so,
• we can view the dump in the mitmproxy ui.

This may end up being superior to the current tcpdump form, which requires sudo and is not HTTP specific (so is clunkier to see in Wireshark).