Add module discovery and autoloading to expect_tests.

scripts/slave/unittests/expect_tests/serialize.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import difflib
 import json
 import os
 import pprint
 
 from collections import OrderedDict
@@ skipping 61 matching lines @@
   """
   @type test: Test()
   @returns: The deserialized data (or NonExistant), and a boolean indicating if
             the current serialized data is in the same format which was
             requested by |test|.
   @rtype: (dict, bool)
   """
   for ext in sorted(SUPPORTED_SERIALIZERS, key=lambda s: s != test.ext):
     path = test.expect_path(ext)
     if path is None:
-      return None, True
+      return NonExistant, True
 
     if ext not in SERIALIZERS and ext == test.ext:
       raise Exception('The package to support %s is not installed.' % ext)
     if os.path.exists(path):
       try:
         with open(path, 'rb') as f:
           data = SERIALIZERS[ext][0](f)
       except ValueError as err:
         raise ValueError('Bad format of %s: %s' % (path, err))
       return data, ext == test.ext
@@ skipping 25 matching lines @@
     return new
   if old == new:
     return None
   else:
     return list(difflib.context_diff(
         pprint.pformat(old).splitlines(),
         pprint.pformat(new).splitlines(),
         fromfile='expected', tofile='current',
         n=4, lineterm=''
     ))