Create testing handbook

[evan-forbes]

I think an overarching goal around our testing should be to put together some written artefact across the teams which can really act as a handbook making it clear for any team member or for anyone interested what we test, how we test it, when we test things and so forth. There is no monolithic one solution fits all kind of approach to testing. It really requires a myriad of different solutions that juggle the tradeoffs between different forms of coverage, "introspectability", time and resource costs. Thinking about testing the same systematic way we think about protocols and writing "specifications" will produce more reliable software. Having a place where people can read about everything will make it easier for us to cover our blindspots.

Originally posted by @cmwaters in #1256 (comment)

while this will be an ongoing endeavor, to close this issue we should at least start it and complete all the necessary documentation for the tests in core/app.