Trace stdout from test-proxy

[heaths]

This is a big refactor of the test-proxy wrapper that will precede other work. This will help diagnose issues that, in the past, you could only do if you started the test-proxy separately and remembered to pass PROXY_MANUAL_START=true when testing.

[heaths]

I've been wanting to do this for a while because it would've helped a couple weeks ago trying to figure out why playback wasn't working in certain circumstances. It was only once I ran the test-proxy separate and watched its logs I could tell; however, interspersing stdout from test-proxy - which cargo test would swallow anyway - would make things more difficult. So, instead, I capture its stdout - which I already was to only get the version and port binding - and trace it all!

I also added a troubleshooting section to the azure_core_test README.md that explains a bit. Basically, by tracing this information it provides a lot of opportunity to customize what level of information - and from where - you want to see. For example, hyper writes a lot of trace information you probably rarely want to see, if ever...so don't!