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Visual Studio 2012 RTM: Async tests are not found when using .NET Framework 4.0

For a project I created some async unit tests inside Visual Studio 2012 RC. 
[TestMethod]  
public async Task AsyncTest()   
{   
 var result = await MyOperationAsync();   
 Assert.IsTrue(result);   
}

Last week I took the opportunity to upgrade to the VS 2012 RTM version. But when I tried to run my async tests again, no tests were executed and the MsTest engine reported that no tests were found?! When I removed the async and await keywords, MsTest found my tests again… What has changed in Visual Studio that this no longer works?

It took me some time to figure out the root cause. What happened?

As the projects I wanted to test were .NET 4.0 projects, I decided to set the Unit Test project also to .NET 4.0. Of course in .NET 4.0, you don’t have async support out-of-the-box. So I installed the “Async targeting pack for Visual Studio 2012”.

The "Async Targeting Pack for Visual Studio 2012" enables projects targeting .NET Framework 4.0 or Silverlight 5 to use the Async language feature in C# 5 and Visual Basic 11. This pack requires Visual Studio 2012 and will not work with Visual Studio 2010.

The pack contains the API support necessary to use the 'async' and 'await' keywords in C# 5.0 and Visual Basic 11, as well as a set of Task-based adapter APIs that allow using some of the existing asynchronous APIs with the new language keywords.
 
This worked fine in Visual Studio 2012 RC. MsTest was able to discover and run my tests without a problem. The moment I upgraded to the RTM version, it no longer worked.
How did I fix it?
After changing the target framework for my Test project to .NET 4.5, my tests were discovered again. Anyone with a better solution?
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