Use unittest2 in all tests

This allows using features such as TestCase,assertIsInstance on Python
2.6 and earlier.

tests/call_function_test.py

 import logging
 import time
-import unittest
+
+import unittest2 as unittest
 
 import mitogen.core
 import mitogen.master

tests/channel_test.py

-import unittest
+import unittest2 as unittest
 
 import mitogen.core
 import testlib

tests/fakessh_test.py

 
 import os
 import shutil
-import unittest
+
+import unittest2 as unittest
 
 import mitogen.fakessh
 

tests/first_stage_test.py

 
 import subprocess
-import unittest
+
+import unittest2 as unittest
 
 import mitogen.master
 import testlib

tests/id_allocation_test.py

 
-import unittest
+import unittest2 as unittest
 
 import testlib
 import id_allocation

tests/importer_test.py

@@ -2,11 +2,11 @@
 import email.utils
 import sys
 import types
-import unittest
 import zlib
 
 import mock
 import pytest
+import unittest2 as unittest
 
 import mitogen.core
 import testlib

tests/local_test.py

 
 import os
-import unittest
+
+import unittest2 as unittest
 
 import mitogen
 import mitogen.ssh

tests/master_test.py

 
 import subprocess
 import time
-import unittest
+
+import unittest2 as unittest
 
 import testlib
 import mitogen.master
@@ -15,7 +16,7 @@ class ScanCodeImportsTest(unittest.TestCase):
         self.assertEquals(list(self.func(co)), [
             (-1, 'subprocess', ()),
             (-1, 'time', ()),
-            (-1, 'unittest', ()),
+            (-1, 'unittest2', ()),
             (-1, 'testlib', ()),
             (-1, 'mitogen.master', ()),
         ])

tests/module_finder_test.py

 import inspect
-import unittest
+
+import unittest2 as unittest
 
 import mitogen.master
 

tests/nested_test.py

 import os
-import unittest
+
+import unittest2 as unittest
 
 import testlib
 

tests/responder_test.py

 
 import mock
 import subprocess
-import unittest
 import sys
 
+import unittest2 as unittest
+
 import mitogen.master
 import testlib
 

tests/select_test.py

 
-import unittest
+import unittest2 as unittest
+
 import mitogen.master
 
 import testlib

tests/ssh_test.py

-
-import unittest
-
 import mitogen
 import mitogen.ssh
 import mitogen.utils
 
+import unittest2 as unittest
+
 import testlib
 import plain_old_module
 

tests/testlib.py

@@ -5,9 +5,10 @@ import re
 import socket
 import sys
 import time
-import unittest
 import urlparse
 
+import unittest2 as unittest
+
 import mitogen.master
 if mitogen.is_master:  # TODO: shouldn't be necessary.
     import docker

tests/utils_test.py

 #!/usr/bin/env python
 
-import unittest
+import unittest2 as unittest
 
 import mitogen.master
 import mitogen.utils