nunit: TestContext does not flow in async method

Example tests recreating issue:

[TestFixture]
public class TestContext_async_issue
{
    [Test]
    public async Task TestContext_Out_should_flow_with_async_execution()
    {
        var expected = TestContext.Out;
        await Task.Yield();
        Assert.AreSame(expected, TestContext.Out);
    }

    [Test]
    public async Task TestContext_CurrentContext_should_flow_with_async_execution()
    {
        var expected = TestContext.CurrentContext;
        await Task.Yield();
        Assert.AreSame(expected, TestContext.CurrentContext);
    }
}

The side effect of that behaviour is that it is not possible to use TestContext.WriteLine() after await as it causes NullReferenceException.

Tested with NUnit 3.5.0, net46 and netcoreapp1.0