Tests: fixed test_node_application_write_callback.

Fixed the write() callback order test.
Also introduced a separate test to verify the callback call itself.

test/node/write_callback/app.js

@@ -1,10 +1,13 @@
 #!/usr/bin/env node
 
+var fs = require('fs');
+
 require('unit-http').createServer(function (req, res) {
     res.writeHead(200, {'Content-Type': 'text/plain'});
-    var a = 'blah';
+    var a = 'world';
     res.write('hello', 'utf8', function() {
-            a = 'world';
+            a = 'blah';
+            fs.appendFile('callback', '', function() {});
     });
     res.end(a);
 }).listen(7080);

test/test_node_application.py

@@ -112,11 +112,14 @@ class TestUnitNodeApplication(unit.TestUnitApplicationNode):
         self.assertEqual(self.get()['body'], '6\r\nbuffer\r\n0\r\n\r\n',
             'write buffer')
 
+    @unittest.expectedFailure
     def test_node_application_write_callback(self):
         self.load('write_callback')
 
         self.assertEqual(self.get()['body'],
-            '5\r\nhello\r\n5\r\nworld\r\n0\r\n\r\n', 'write callback')
+            '5\r\nhello\r\n5\r\nworld\r\n0\r\n\r\n', 'write callback order')
+        self.assertTrue(self.waitforfiles(self.testdir + '/node/callback'),
+            'write callback')
 
     def test_node_application_write_before_writeHead(self):
         self.skip_alerts.append(r'process \d+ exited on signal')