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.

首先审查现有的 Testing framework，以及 .NET 中其他地方如何测试 async 和 IObservable 函数。将这些方法与 Excel-DNA 在自动和手动计算模式下的行为进行比较；完成的标准是形成一份有文档记录的测试策略，并明确决定是否可以调整 helpers。

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