RX + TPL + Testing

I currently struggeling with the issue testing a TPL function inside of a RX statement. The problem is also that I don't have access to the observable.

Here is my test:

[Test]
public void TestRaceConditions()
{
    // Arrange
    TestScheduler testScheduler = new TestScheduler();
    List<string> source = new List<string> {"Hello"};
    List<string> results = new List<string>();

    source.ToObservable().SelectMany(v => Task.Run(() => 
    {
        Thread.Sleep(10000); // Simulate long running operation
        return v;
    }).ToObservable(testScheduler))
    .ObserveOn(testScheduler)
    .Subscribe(s => results.Add(s));

    // Act
    testScheduler.Start();

    Assert.That(results.Count, Is.EqualTo(1));
    Assert.That(results[0], Is.EqualTo("Hello"));
}

I've googled around and found many articles and advices like:

• Do not mix TPL and RX
• Create a wrapper for the TPL code where you can mock and return a completed task. ==> But what about integration tests?

What I also tried is to create a custom TaskScheduler, but then I have to replace every Task.Run with Task.Factory.StartNew.

So the question is:

• How to get control over the task or the observable in the test just with Schedulers that I know when all elements are processed and finished?