Add documentation on writing tests

[jdavcs]

We need documentation on how to write tests (simple walk-throughs with basic examples).

Documentation should include:

• [x] instructions on how to choose what types of tests to write in different scenarios
• [ ] sample unit tests + instructions
• [x] sample integration tests + instructions
• [ ] sample API tests + instructions (unless we group them with integration tests)
• [ ] sample selenium tests + instructions
• [x] link to client tests documentation in client/readme.md
• [ ] ...anything else I'm missing

After (most of) this documentation has been posted, I think we can try to require writing accompanying tests for (most?) PRs (like in SQLAlchemy, CPython and many, many others, I'm sure). Or, at least, change the wording of this line in contributing.md to make it "less optional".

[Nerdinacan]

If you could please add:

How to configure an IDE to set a breakpoint in a selenium test so you can like..... do something useful with that test when a failure is discovered.