bisect-kit: refactoring session dir

Hide session_base variable from users and unify how session_dir is
determined.

This CL also avoid touch default session folder during unit test.

BUG=None
TEST=unittest

Change-Id: If722a185681d177a646cdaf6ee3b52e299f987eb
diff --git a/bisect_git_test.py b/bisect_git_test.py
index fe7bd20..2b9bb74 100644
--- a/bisect_git_test.py
+++ b/bisect_git_test.py
@@ -12,9 +12,11 @@
 
 from bisect_kit import bisector_cli
 from bisect_kit import git_util_test
+from bisect_kit import testing
 import bisect_git
 
 
+@testing.mock_session_base()
 @mock.patch('bisect_kit.common.config_logging', mock.Mock())
 class TestGitDomain(unittest.TestCase):
   """Test GitDomain class."""