Create testing strategy document for this project

[stark525]

Currently, there is an example test documented using a payload.json object, which is effectively an integration test.

It would be great to provide standards for testing cirrus tasks for development purposes, e.g. TDD.

I'm thinking we could have a middleware testing layer to mock SNS/SQS/workflow - and convert a cirrus object to STAC record. This will allow us to unit test the handler.

Also adding pytest as the unit test framework of choice and examples for testing other business logic.