docs: Add testing overview

[thanethomson]

We need an overview of what and how we're testing different crates in the repo. It should probably be in the README in the root of the repository. This should ideally include:

• A brief description of how to run all tests locally
• A high-level, brief overview of the different testing approaches we use
• Links to the sections in the READMEs in each crate that describe that crate's testing approach (once #595, #596 and #597 are done).