Do you know the MsTest.exe is 32 bit by default. MS is yet to come up with 64 bit version of this exe, to run the tests in 64 bit mode.
Found the following steps in one of the useful post on Stackoverflow: http://stackoverflow.com/questions/5880337/run-tests-in-64-bit
1.Backup mstest.exe and run CorFlags.exe MSTest.exe /32BIT- /Force to remove the 32-bit flag
2.Run your tests with /noisolation: MSTest.exe /testcontainer: /resultsfile: /noisolation
Tried validate the 3 step from post on my windows 7 machine but could not be validated so skipping that.