Shadir information can not be overwritten.

It must be possible to append information on shadir files, it must
simulate the real server. Otherwise there is no way to test the behavior
using different signatures.

src/zc/buildout/testing.py

@@ -548,6 +548,17 @@ class NCHandler(SimpleHTTPServer.SimpleHTTPRequestHandler):
             pass
         else:
             data = json.dumps([d])
+            if os.path.exists(path) and 'shadir' in path:
+                f = open(path, 'r')
+                try:
+                    file_data = f.read()
+                finally:
+                    f.close()
+                file_data = file_data.strip()
+                json_data_list = json.loads(file_data)
+                json_data_list.append(d)
+                data = json.dumps(json_data_list)
+
         open(path, 'wb').write(data)
         self.send_response(201)