arti-testing: Initial implementation

This commit adds a new program to try to implement the ideas behind experimentation in #329 (closed). In particular, it tries to implement basic client "can I bootstrap and connect" functionality testing, with a lot of instrumentation, and support for breaking things.

So far, the instrumentation is limited to counting TCP bytes and connections, and counting events. Still, this is enough to measure behavior on some of the incorrect-clock tests.

NOTE:

For now, you are required to pass in an explicit configuration, in hopes that this will lead you to override your storage directories for doing specific experiments.