BizTalk Unit testing Code coverage

Hi All , 

We are using BizTalk 2013 . All of our applications have most of the logic written in maps and the maps are 90% custom xslt’s.

We have started using Visual Studio 2012  unit testing frame work to write unit tests for each of the map .

Example is as below

 public void POToInvoiceMapTest()
        {
            POToInvoice target = new POToInvoice();

            //=== Use the HelloWorld sample directory path for the message files ===//

            string strSourcePO_XML = testContextInstance.TestDir + "..\\..\\..\\SamplePOInput.xml";
            string strDestInvoice_XML = testContextInstance.TestDir + "..\\..\\..\\SampleInvoiceOutput.xml";


            //=== Test the map by using the TestMap method of the TestableMapBase class ===//

            target.ValidateOutput = true;
            target.TestMap(strSourcePO_XML,
                           Microsoft.BizTalk.TestTools.Schema.InputInstanceType.Xml,
                           strDestInvoice_XML,
                           Microsoft.BizTalk.TestTools.Schema.OutputInstanceType.XML);

            
            //=== Output file should be created as a result of testing the map ===//

            Assert.IsTrue(File.Exists(strDestInvoice_XML)); 
        }

The tests usually run fine , but we want to analyze the code coverage for the tests . When we run the analyze code coverage the code coverage that is shown is that of the Unitest.cs class and not for the actual xslt .

Usually C#.net the code coverage analysis shows the class library or helper which is called ,its code coverage .

How do we measure the code coverage for map unit tests in BizTalk ?

Any help would be really appreciated !!!

Thanks