arti-testing: support for simulated TCP breakage

This is the second part of my work on arti-testing, to help simulate failures for #329 (closed).

This branch makes it possible for TCP connections to fail with an IO error, time out, or succeed but ignore all data. These failures can be enabled immediately, after a delay, or in a particular stage. Failures can be set up for all TCP connections, IPv4 only, IPv6 only, or non-443.