List existing tests

[hug-dev]

As a proof to the following mitigation on the Client threat model:

The Parsec client is coded with safety in mind and is tested extensively.

There should be a list of tests, somewhere, describing in high-level terms the kind of tests that we do on the client.

This could either be in the book or somewhere in this repo. It would also probably be good to have a link from this repo to the threat model. Clients using Parsec through this Rust crate directly might not even look at the global Parsec documentation but just want to use it.