Test that call is awaited using NUnit and Moq

I have a piece of production code where some long lasting task must finish before something else can be done (See example below). I am able to test the call order but not the await (which is missing in the example). What must I do to test this?

using System.Threading.Tasks;
using Moq;
using NUnit.Framework;

namespace TestingCallsAwaitedDemo
{
    public interface IFoo
    {
        Task DoThisFirst();
    }

    public interface IBar
    {
        void ThenDoThis();
    }

    public class Demo
    {
        private readonly IFoo _foo;
        private readonly IBar _bar;

        public Demo(IFoo foo, IBar bar)
        {
            _bar = bar;
            _foo = foo;
        }

        public async Task DoSomething()
        {
            _foo.DoThisFirst(); // The await is missing. How do I force its presence with a test?
            _bar.ThenDoThis();
        }
    }

    [TestFixture]
    public class DemoTests
    {
        private Demo classUnderTest;
        private Mock<IFoo> fooMock;
        private Mock<IBar> barMock;

        [SetUp]
        public void Setup()
        {
            fooMock = new Mock<IFoo>();
            barMock = new Mock<IBar>();
            classUnderTest = new Demo(fooMock.Object, barMock.Object);
        }

        [Test]
        public async Task MustFinishDoThisFirstBeforeCallingThenDoThis()
        {
            var callOrder = string.Empty;
            fooMock.Setup(v => v.DoThisFirst())
                .Callback(() => callOrder += "DoThisFirst >> ");
            barMock.Setup(v => v.ThenDoThis())
                .Callback(() => callOrder += "ThenDoThis >> ");

            await classUnderTest.DoSomething();

            Assert.AreEqual(
                "DoThisFirst >> ThenDoThis >> ",
                callOrder);

            //How do I test the missing await?
        }
    }
}