releasetools: Add a common base class for unittests.

All the releasetools unittests extend the common base class of
test_utils.ReleaseToolsTestCase. Define tearDown() in the base class to
do the clean-up works.

Test: `pylint --rcfile=pylintrc test_*.py`
Test: `python -m unittest discover .`
Change-Id: I51775d964ef032dcdf3bb89c55e1a31371cde708
diff --git a/tools/releasetools/test_sign_target_files_apks.py b/tools/releasetools/test_sign_target_files_apks.py
index ac1b567..18762ee 100644
--- a/tools/releasetools/test_sign_target_files_apks.py
+++ b/tools/releasetools/test_sign_target_files_apks.py
@@ -14,11 +14,8 @@
 # limitations under the License.
 #
 
-from __future__ import print_function
-
 import base64
 import os.path
-import unittest
 import zipfile
 
 import common
@@ -28,7 +25,7 @@
     ReplaceVerityKeyId, RewriteProps)
 
 
-class SignTargetFilesApksTest(unittest.TestCase):
+class SignTargetFilesApksTest(test_utils.ReleaseToolsTestCase):
 
   MAC_PERMISSIONS_XML = """<?xml version="1.0" encoding="iso-8859-1"?>
 <policy>
@@ -39,9 +36,6 @@
   def setUp(self):
     self.testdata_dir = test_utils.get_testdata_dir()
 
-  def tearDown(self):
-    common.Cleanup()
-
   def test_EditTags(self):
     self.assertEqual(EditTags('dev-keys'), ('release-keys'))
     self.assertEqual(EditTags('test-keys'), ('release-keys'))