evaluate check item in validator.

        Args:
            check_item: check_item should only be the following 5 formats:
                1, variable reference, e.g. $token
                2, function reference, e.g. ${is_status_code_200($status_code)}
                3, dict or list, maybe containing variable/function reference, e.g. {"var": "$abc"}
                4, string joined by delimiter. e.g. "status_code", "headers.content-type"
                5, regex string, e.g. "LB[\d]*(.*)RB[\d]*"

            resp_obj: response object

def __eval_validator_check(self, check_item, resp_obj):
        """ evaluate check item in validator.

        Args:
            check_item: check_item should only be the following 5 formats:
                1, variable reference, e.g. $token
                2, function reference, e.g. ${is_status_code_200($status_code)}
                3, dict or list, maybe containing variable/function reference, e.g. {"var": "$abc"}
                4, string joined by delimiter. e.g. "status_code", "headers.content-type"
                5, regex string, e.g. "LB[\d]*(.*)RB[\d]*"

            resp_obj: response object

        """
        if isinstance(check_item, (dict, list)) \
            or isinstance(check_item, parser.LazyString):
            # format 1/2/3
            check_value = self.eval_content(check_item)
        else:
            # format 4/5
            check_value = resp_obj.extract_field(check_item)

        return check_value