First, lets look at the easiest way to deploy additional files. If you did just want everything at the root folder you can use the Test menu and select option for editing the configuration as shown in the image. This will bring up the test configuration dialog where you can use the Add File button and navigate to the file you want to add. Don't forget to show all files as demonstrated in the image.
When you want to deploy to a different folder you have to use the DeploymentItem attribute. It takes a second parameter that allows you to specify a subfolder where you want the item deployed. In my case I just want to maintain the same relative folder path so that I don't need to change code just to make my unit tests work.
Here is the same example file being added to an XmlData subfolder using the DeploymentItem attribute method.