Support running single-threaded async tests on Linux

[MCMrARM]

Currently the [Apartment(ApartmentState.STA)] annotation is not supported on Linux, which means that the SingleThreadedSynchronizationContext cannot be enabled on Linux and tests using the annotation are skipped with "Apartment state cannot be set on this platform.". There is as far as I can see no other way to enable the nunit's SingleThreadedSynchronizationContext. It also seems that MessagePumpStrategy only has hardcoded strategies for the default SynchronizationContextes and does not support a custom SynchronizationContext, making use of a custom single threaded SynchronizationContext impossible.

[manfred-brands]

Here is a base class you can use until we have a more permanent solution:

public abstract class SynchronizedTestFixture
{
    private SynchronizationContext? _previousContext;
    private SynchronizationContext? _ourContext;

    [SetUp]
    public void SynchronizedSetup()
    {
        _previousContext = SynchronizationContext.Current;
        _ourContext = CreateNUnitSynchronizationContext();
        SynchronizationContext.SetSynchronizationContext(_ourContext);
    }

    [TearDown]
    public void SynchronizedTeardown()
    {
        SynchronizationContext.SetSynchronizationContext(_previousContext);
    }

    private static SynchronizationContext CreateNUnitSynchronizationContext()
    {
        Type type = typeof(Assert).Assembly.GetType("NUnit.Framework.Internal.SingleThreadedTestSynchronizationContext")!;

        return (SynchronizationContext)Activator.CreateInstance(type,
            BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Instance,
            null,
            [new TimeSpan(TimeSpan.TicksPerSecond)],
            null)!;
    }
}

@OsirisTerje There is the existing SingleThread attribute which works for non-async tests.
Could we call this a bug and change the code that if SingleThread is set we also set a SingleThreadedTestSynchronizationContext?

[OsirisTerje]

Yes, that makes sense to me.