Fixed expand_path to use working directory set by chdir

`File.expand_path` with one argument should expand from the current working directory. However, it seems that it keeps using the working directory of the real file system instead of the fake one set by `Dir.chdir`. I added a test for this and updated the code, but I am unsure of the impact of my changes. The test suite says I haven't broken anything, so that's a good sign.