Add tools for testing connectivity handling

[agronholm]

Things to check first

• I have searched the existing issues and didn't find my feature already requested there

Feature description

I'm thinking of adding some sort of MiddleMan class which works as both listener and connector. Once it receives a connection, it would connect to the final destination (some service being tested, probably). The test code could then arbitrarily disconnect either, or both sides, or make them unresponsive to test how the code would fare in these situations.

Testing TLS connections like this is a tad more complicated, but completely doable.

This would be best paired with #151.

Use case

It would be prudent for implementations of network protocols to be tested around their handling of disconnections, and this is not trivial to accomplish with existing tools.