test: add failing http `agent: null` test

See #7012.

test/simple/test-http-agent-null.js

@@ -0,0 +1,47 @@
+// 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 net = require('net');
+
+var request = 0;
+var response = 0;
+process.on('exit', function() {
+  assert.equal(request, 1, 'http server "request" callback was not called');
+  assert.equal(response, 1, 'http request "response" callback was not called');
+});
+
+var server = http.createServer(function(req, res) {
+  request++;
+  res.end();
+}).listen(function() {
+  var options = {
+    agent: null,
+    port: this.address().port
+  };
+  http.get(options, function(res) {
+    response++;
+    res.resume();
+    server.close();
+  });
+});