Adds stdout and stderr if present.

def _fill_out_err(result, testcase):
        """Adds stdout and stderr if present."""
        if result.get("stdout"):
            system_out = etree.SubElement(testcase, "system-out")
            system_out.text = utils.get_unicode_str(result["stdout"])

        if result.get("stderr"):
            system_err = etree.SubElement(testcase, "system-err")
            system_err.text = utils.get_unicode_str(result["stderr"])