Testing Utility: FakeScheduler

[JakenVeina]

Added a new implementation of IScheduler to allow full control over execution of scheduled actions, by test orchestration code.

Needed this to be able to simulate inaccuracies in real-world schedulers, for intervals around 1ms, in particular with reference to #716.

[dwcullop]

I don't have an issue with this, but I'd like to understand the need better. The regular test scheduler allows for control down to the Tick (100ns). What does this give us that the other doesn't?

A replacement Test Scheduler should also have the time control methods the other one does, like AdvanceTo and AdvanceBy (not sure if those are exactly the names).

[JakenVeina]

The test secheduler does not allow simulation of inaccuracies. It will always invoke scheduled operations at exactly the (virtual) time they were scheduled for.

This class does not have the .AdvanceTo() the .AdvanceBy() methods because those APIs couple together control over time simulation and action invokation, and the need this fills is specifically to control those things separately.

This class is not intended to replace the existing TestScheduler, it is intended to support edge cases that TestScheduler cannot.

[JakenVeina]

I've identified the build failure as an un-related faulty test, which intermittently fails, and documented it as #817.

[RolandPheasant]

I won't know the difference this makes until you use it. However it's for testing, so let's get it in.

[github-actions]

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