test: fix test-fs-symlink-dir-junction-relative

  * The test no longer relies on being invoked from a particular
    working directory to function properly.
  * fs.symlink() and fs.symlinkSync() are both tested.
  * The test now cleans up after itself.

This commit fixes https://github.com/iojs/io.js/issues/126

R=@bnoordhuis