DeploymentItem not deploying files

I am using MS unit testing framework for testing my C# library. I have to open a file which I'm deploying using DeploymentItem attribute. But it is not deploying file to the Test deployment directory.

In my Unit-test project, I created a folder TestFile in this folder there are multiple files, lets say a.txt, b.txt.

Now in my Unit-test class I added DeploymentItem attribute. But file is not being copied.

Here is my code.

[TestClass]
[DeploymentItem("TestFile")]
public class CustomLibraryTest
{
    public TestContext TestContext { get; set; }

    [TestMethod]
    [DeploymentItem(@"TestFiles\a.txt")]  // THis should deploy a.txt to test deployment directory
    {
        var path = TestContext.TestDeploymentDir + "a.txt";
        // Now when I debug this unit-test, and open path in explorer,
        // a.txt is not present in "TestResults\Deploy_fhafeez 2013-05-28 13_02_37\Out" folder
    }
}

What I am doing wrong?