DeploymentItem is a bit of a mess.
Each file in your solution will have a “Copy To Output Folder” setting in VS.NET. You need this to be “Copy Always” (or similar) in order to get the files into the output folder.
Check that you’ve got this set for the new files. If you don’t have this set then the files won’t get copied to the output folder, and then they can’t be deployed from the output folder to the folder where MSTest does it stuff.
Personally, if I have files that I need for my unit tests I’ve found that embedding those files as resources into an assembly, and having that assembly “unpack” itself during the tests is a more predictable way of doing things. YMMV.
note: These comments are based upon my experience with VS2010. Comments to my answer would suggest that this is not problem with VS2012. I still stand by comments that using embedded resources involves less “magic” and, for me, makes the “arrange” stage of my unit tests much more explicit.