What are the benefits of asynchronous unit tests?

I'm new to the async/await world and I'm trying to figure out the benefits of writing asynchronous unit tests for async methods. That is, must a unit test for an async method call that async method asynchronously? If it calls the async method synchronously using Task.Run(), what is lost? In the latter case, code coverage isn't impacted as far as I can see.

The reason I'm asking this is because our mocking software (we use TypeMock) can't support async/await. (They say there is a legitimate reason for this lack of support, and I don't disagree with them.) By calling the async methods synchronously in the unit tests, we can workaround this issue. However, I'd like to know whether we are cutting any corner by doing this.

For example, let's say I have the following async method:

public async Task<string> GetContentAsync(string source)
{
    string result = "";
    // perform magical async IO bound work here to populate result
    return result;
}

The following is the ideal unit test which doesn't work:

[TestMethod()]
public async Task GetContentAsyncTest()
{
    string expected = "thisworks";
    var worker = new Worker();
    // ...mocking code here that doesn't work!
    string actual = await worker.GetContentAsync();
    Assert.AreEqual(expected, actual);
}

But this works, and it does provide the code coverage we need. Is this OK?

[TestMethod()]
public void GetContentAsyncTest()
{
    string expected = "thisworks";
    var worker = new Worker();
    // mocking code here that works!
    string actual = Task.Run(() => worker.GetContentAsync()).Result;
    Assert.AreEqual(expected, actual);
}