Unable to see NUnit Assert message output

I’m using the Unity Test Tools asset and I’ve written a unit test. I have a pesky assertion that’s failing and I want to print a message with it to help me figure out why the assertion is failing. But I can’t seem to get any output to show up from the messages. I have tried something basic like this:

Assert.Fail("Hello world!");

It makes the test case fail, but the message doesn’t show up under Test Results/Output where I’d expect it to be. I’m running Unity v5.1.3f1 on OSX.

Works fine in Unity 5.3.4f1/Windows 8 and Unitys built in runner.

using NUnit.Framework;

public class NewEditorTest
{
    [Test]
    public void EditorTest()
    {
        Assert.Fail("Hello world!");
    }
}