def relative_test_filename(self, filename):
        """Returns a Unix-style path for a filename relative to web_tests.

        Ports may legitimately return absolute paths here if no relative path
        makes sense.
        """
        # Ports that run on windows need to override this method to deal with
        # filenames with backslashes in them.
        if filename.startswith(self.web_tests_dir()):
            return self.host.filesystem.relpath(filename, self.web_tests_dir())
        else:
            return self.host.filesystem.abspath(filename)