benchmark: chunky http client should exit with 0

Previously when there is an error in the chunky client of the
http benchmark, the server would not check the exit code and
thus produce invalid results.

PR-URL: https://github.com/nodejs/node/pull/12916
Fixes: https://github.com/nodejs/node/issues/12903
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

benchmark/http/http_server_for_chunky_client.js

@@ -1,5 +1,6 @@
 'use strict';
 
+var assert = require('assert');
 var path = require('path');
 var http = require('http');
 var fs = require('fs');
@@ -37,6 +38,7 @@ server.listen(PIPE);
 
 var child = fork(pep, process.argv.slice(2));
 child.on('message', common.sendResult);
-child.on('close', function() {
+child.on('close', function(code) {
   server.close();
+  assert.strictEqual(code, 0);
 });