Review all tests that use Task.Delay()

[iancooper]

Is your feature request related to a problem? Please describe.
We use Task.Delay in a number of tests related to the work of the message pump. The problem we have is that the pump runs until sent a terminate, and this takes time to happen. But, tests that use a Task.Delay are fragile. Fragile Tests cause us to lose our rhythm investigating them and may encourage us to work with red tests.

Describe the solution you'd like
No plan as yet, but it it worth understanding if there is a design issue here. I have no desire to change our message pump, which is key to how we build Brighter, but... it is possible that we need to look at ideas that help us test behavior without being at the mercy of the pump. Options include a replaceable pump that can be swapped out when we are not testing the pump itself, that behaves in a more controllable fashion, or imperative shell, functional core style approaches.

Describe alternatives you've considered
Rewriting the pump using something like cancellation tokens. Having seen how painful this was projects like JustSaying, I am confirmed that our choice to use a 'classic' message pump design continues to pay dividends.

Additional context
DispatcherConnectionTests are a typical example of a Fragile Test related to timings.