Improve use as a library

[howardjohn]

Huge fan of this tool - thanks for providing it!

I was interested in additionally using it as a library. This sort of works - I can easily import it and run a client and server. However, a few tweaks would make it a bit more ergonomic to use.

• Opts: probably could make the client not require server params and vis-versa; neither should need to depend on verbose
• Server: allow a way to bind to port 0 (random) then get the real port
• Server: expose when bound to port. Currently its hard to do run_server() and then know when its safe to run the client
• sub-second granularity of test runs would be nice
• return a TestResults from run_client for programmatic access to the results