Document our philosophy on Testing

[brad-decker]

Suggested sections:

1. Unit vs Integration vs E2E
2. How should we approach unit testing?
3. Mocking/Spying
4. Snapshot philosophy
5. Tooling (jest, etc)

Essentially give as much background into our current thinking around tests so that a new engineer is effective at reading existing tests as well as writing new ones.

[Gudahtt]

Documentation was added for 1. in #99.

We have covered 2 and 5 adequately well in earlier changes as well.

There are still some additions that I'd like to make about 3 and 4. I have captured the remaining work on these topics in #114 and #115