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_validate_target_files.py b/tools/releasetools/test_validate_target_files.py
index ecb7fde..7b29ef8 100644
--- a/tools/releasetools/test_validate_target_files.py
+++ b/tools/releasetools/test_validate_target_files.py
@@ -16,12 +16,9 @@
 
 """Unittests for validate_target_files.py."""
 
-from __future__ import print_function
-
 import os
 import os.path
 import shutil
-import unittest
 
 import build_image
 import common
@@ -29,14 +26,11 @@
 from validate_target_files import ValidateVerifiedBootImages
 
 
-class ValidateTargetFilesTest(unittest.TestCase):
+class ValidateTargetFilesTest(test_utils.ReleaseToolsTestCase):
 
   def setUp(self):
     self.testdata_dir = test_utils.get_testdata_dir()
 
-  def tearDown(self):
-    common.Cleanup()
-
   def _generate_boot_image(self, output_file):
     kernel = common.MakeTempFile(prefix='kernel-')
     with open(kernel, 'wb') as kernel_fp: