Introduce `libp2p-swarm-test` crate

[thomaseizinger]

Description

Introduce a crate that contains reusable bits that make it easy to correctly test NetworkBehaviour implemenations. In particular, the crate should support the setting up and connecting a number of swarms, driving them correctly and asserting the emitted events.

Motivation

For code to be high quality it should be tested. If testing is easier, users are more likely to do it, thus yielding higher quality code.

Current Implementation

Are you planning to do it yourself in a pull request?

Yes.