Как получить путь элемента в lxml?

используйте DeploymentItem атрибут

using System;
using System.IO;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using CarMaker;

namespace DeploymentTest
{
    [TestClass]
    public class UnitTest1
    {
        [TestMethod()]
        [DeploymentItem("testFile1.xml")]
        public void ConstructorTest()
        {
            string file = "testFile1.xml";
            Assert.IsTrue(File.Exists(file), "deployment failed: " + file +
                " did not get deployed");
        }
    }
}