public static String httpJsonResponseString;
 HTTPResponse res_usage;
    if(!Test.isRunningTest()){
        res_usage = http_usage.send(req);
    }else{
        HttpResponse res = new HttpResponse();
        res.setHeader('Content-Type', 'application/json');
        res.setBody(httpJsonResponseString);
        res.setStatusCode(200);
        res_usage = res;
    }