Offer a single-threaded SynchronizationContext implementation #323
Description
Is your feature request related to a problem? Please describe.
On .NET Core there is no single-threaded SynchronizationContext to use, thereby making JoinableTaskFactory.SwitchToMainThreadAsync() no-op. Even .NET Core 3.x won't help everyone since they may be running on non-Windows platforms.
Also when unit testing a library that runs in a GUI app, it requires some significant setup to emulate a main thread.
Describe the solution you'd like
We can offer a single-threaded SynchronizationContext as a public class in the library and document best practices for testing that uses it.
This new class would also be available for use in non-testing processes on non-Windows platforms for folks who want to use it.
Additional context
More discussion on this with @weltkante and @sharwell is available in #322