Stopped using django.utils.unittest in the test suite.

Refs #20680.

docs/topics/testing/advanced.txt

@@ -37,7 +37,7 @@ Example
 
 The following is a simple unit test using the request factory::
 
-    from django.utils import unittest
+    import unittest
     from django.test.client import RequestFactory
 
     class SimpleTest(unittest.TestCase):

docs/topics/testing/overview.txt

@@ -800,7 +800,7 @@ Example
 
 The following is a simple unit test using the test client::
 
-    from django.utils import unittest
+    import unittest
     from django.test.client import Client
 
     class SimpleTest(unittest.TestCase):
@@ -1143,7 +1143,7 @@ worry about state (such as cookies) carrying over from one test to another.
 
 This means, instead of instantiating a ``Client`` in each test::
 
-    from django.utils import unittest
+    import unittest
     from django.test.client import Client
 
     class SimpleTest(unittest.TestCase):