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

tools/releasetools/test_add_img_to_target_files.py

@@ -16,7 +16,6 @@
 
 import os
 import os.path
-import unittest
 import zipfile
 
 import common
@@ -30,14 +29,11 @@ from rangelib import RangeSet
 OPTIONS = common.OPTIONS
 
 
-class AddImagesToTargetFilesTest(unittest.TestCase):
+class AddImagesToTargetFilesTest(test_utils.ReleaseToolsTestCase):
 
   def setUp(self):
     OPTIONS.input_tmp = common.MakeTempDir()
 
-  def tearDown(self):
-    common.Cleanup()
-
   def _verifyCareMap(self, expected, file_name):
     """Parses the care_map.pb; and checks the content in plain text."""
     text_file = common.MakeTempFile(prefix="caremap-", suffix=".txt")