Why do async unit tests fail when the async/await keywords aren't used?

According to this discussion, there should be no difference between the following two methods:

public async Task Foo()
{
    await DoSomethingAsync();
}

public Task Foo()
{
    return DoSomethingAsync();
}

Actually, it would seem that for very simple methods, the invocation without the async/await keywords would be preferred, as they remove some overhead.

However, this apparently doesn't always work in unit tests.

MSTest

[TestClass]
public class AsyncTest
{
    [TestMethod]
    public async Task Test1()
    {
        await Task.Delay(0);
    }

    [TestMethod]
    public Task Test2()
    {
        return Task.Delay(0);
    }
}

NUnit

[TestFixture]
public class AsyncTest
{
    [Test]
    public async Task Test1()
    {
        await Task.Delay(0);
    }

    [Test]
    public Task Test2()
    {
        return Task.Delay(0);
    }
}

XUnit

public class AsyncTest
{
    [Fact]
    public async Task Test1()
    {
        await Task.Delay(0);
    }

    [Fact]
    public Task Test2()
    {
        return Task.Delay(0);
    }
}

• In all cases, Test1 passes.
• In MSTest, Test2 shows up in the test runner, but it doesn't run.

• In NUnit, Test2 is ignored, with the message:

  Test method has non-void return type, but no result is expected

• In XUnit, Test2 passes.

Since the tasks are still awaitable in all cases, what is it about the async keyword that affects the NUnit and MSTest test runners? Perhaps some reflection issue?