Provide guidance on unit testing orchestrations

Because of the way Durable JS uses generators, it's not very easy to write tests. #49 has some examples here but they're hard to write.

Wonder if we should publish a library to make this easier. Here's an attempt of an API that works for different test frameworks: https://github.com/anthonychu/test-durable-js-testing/blob/master/__tests__/DurableFunctionsOrchestratorJS.tests.js

Thoughts @ConnorMcMahon @cgillum?

Start by reading the examples referenced in #49 and the linked __tests__/DurableFunctionsOrchestratorJS.tests.js attempt. Compare how Durable JS generators are tested across frameworks, then clarify whether the outcome should be guidance or a reusable library. Done would be a documented, agreed testing approach or a defined library proposal.

Written by the indexing model from the issue text.