[test] Add core `test-http-many-keep-alive-connections` test

Modifications:

  * make client connect to `PROXY_PORT` instead of `PORT`
  * `process.exit` instead of `server.close` (process wouldn't exit
    otherwise, I'm not sure why)

test/core/simple/test-http-many-keep-alive-connections.js

+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+var common = require('../common');
+var assert = require('assert');
+var http = require('http');
+
+var expected = 10000;
+var responses = 0;
+var requests = 0;
+var connection;
+
+var server = http.Server(function(req, res) {
+  requests++;
+  assert.equal(req.connection, connection);
+  res.writeHead(200);
+  res.end('hello world\n');
+});
+
+server.once('connection', function(c) {
+  connection = c;
+});
+
+server.listen(common.PORT, function() {
+  var callee = arguments.callee;
+  var request = http.get({
+    port: common.PROXY_PORT,
+    path: '/',
+    headers: {
+      'Connection': 'Keep-alive'
+    }
+  }, function(res) {
+    res.on('end', function() {
+      if (++responses < expected) {
+        callee();
+      } else {
+        process.exit();
+      }
+    });
+  }).on('error', function(e) {
+    console.log(e.message);
+    process.exit(1);
+  });
+  request.agent.maxSockets = 1;
+});
+
+process.on('exit', function() {
+  assert.equal(expected, responses);
+  assert.equal(expected, requests);
+});