Testing - develop a testing strategy for async and real-time streaming functions

RTD-based async and streaming functions are easy to develop with Excel-DNA, but remain hard to test. We need a plan for these and perhaps some helpers in the Testing framework. It would be a start idea to see and document how async and IObservable functions are tested in .NET elsewhere, and see if there's a way to adapt this to the Excel environment. Behaviour of these features under 'manual' calculation mode is also quite different to automatic mode, and can perhaps be some help in knowing when to check values.

Start by reviewing the existing Testing framework and how async and IObservable functions are tested in .NET elsewhere. Compare those approaches with Excel-DNA behavior in automatic and manual calculation modes; done means a documented testing strategy and a clear decision on whether helpers can be adapted.

Written by the indexing model from the issue text.