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);