Disable laziness in ReactiveCommand and ToProperty in unit test runner

Consider the following VM constructor code:

```cs
ToasterList = new ReactiveList<Toaster>();
ToasterList.CollectionCount
    .Select(x => String.Format("{0} toasters!", x))
    .ToProperty(this, x => x.ToasterPrompt, out toasterPrompt);
```

Then the following test:

```cs
viewModel = new MyViewModel();
viewModel.ToasterList.Add(new Toaster());

// Fails! Why????
Assert.Equal("1 toasters!", viewModel.ToasterPrompt);
```

This fails because in RxUI 6.0, `ObservableAsPropertyHelper` is now lazy, and doesn't subscribe to its source until the `Value` is accessed. In the application, this is a good thing - since the person subscribing to us is a View, we don't waste subscriptions on things that aren't being displayed to the user. 

However, in a test runner, we don't need that efficiency, and we're confusing users who make reasonable assertions then are surprised when they don't work