Serialized some tests that interact with the filesystem.

Considering the APIs exercised by these test cases, it's hard to make
them independent.

tests/model_fields/test_imagefield.py

@@ -8,6 +8,7 @@ from django.core.exceptions import ImproperlyConfigured
 from django.core.files import File
 from django.core.files.images import ImageFile
 from django.test import TestCase
+from django.test.testcases import SerializeMixin
 from django.utils._os import upath
 
 try:
@@ -27,11 +28,13 @@ else:
     PersonTwoImages = Person
 
 
-class ImageFieldTestMixin(object):
+class ImageFieldTestMixin(SerializeMixin):
     """
     Mixin class to provide common functionality to ImageField test classes.
     """
 
+    lockfile = __file__
+
     # Person model to use for tests.
     PersonModel = PersonWithHeightAndWidth
     # File class to use for file instances.