bisect-kit: fix unit tests

ContextDecorator shouldn't be used as class decorator. Some unit tests
didn't run because of this mistake.

BUG=None
TEST=unit test

Change-Id: Icfa034d1d41a8e834ae09c4e6f0568a1cc8a3576
diff --git a/bisect_git_test.py b/bisect_git_test.py
index 2b9bb74..f3ae288 100644
--- a/bisect_git_test.py
+++ b/bisect_git_test.py
@@ -16,18 +16,20 @@
 import bisect_git
 
 
-@testing.mock_session_base()
 @mock.patch('bisect_kit.common.config_logging', mock.Mock())
 class TestGitDomain(unittest.TestCase):
   """Test GitDomain class."""
 
   def setUp(self):
+    self.session_base_patcher = testing.SessionBasePatcher()
+    self.session_base_patcher.patch()
     self.git_repo = tempfile.mkdtemp()
     self.git = git_util_test.GitOperation(self.git_repo)
     self.git.init()
 
   def tearDown(self):
     shutil.rmtree(self.git_repo)
+    self.session_base_patcher.reset()
 
   def test_basic(self):
     """Tests basic functionality."""