Wait until all Task finish in unit test

I have this class I want to unit test:

public class SomeClass
{
    public void Foo()
    {
        Bar();
    }

    private void Bar()
    {
        Task.Factory.StartNew(() =>
        {
            // Do something that takes some time (e.g. an HTTP request)
        });
    }
}

And this is how my unit test looks like:

[TestMethod]
public void TestFoo()
{
    // Arrange
    var obj = new SomeClass();

    // Act
    obj.Foo();
    obj.Foo();
    obj.Foo();

    // Assert
    /* I need something to wait on all tasks to finish */
    Assert.IsTrue(...);
}

So, I need to make the unit test thread wait until all tasks started in the Bar method have finished their job before starting my assertions.

IMPORTANT: I cannot change SomeClass

How can I do that?