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_verity_utils.py

@@ -16,25 +16,22 @@
 
 """Unittests for verity_utils.py."""
 
-from __future__ import print_function
-
 import os.path
-import unittest
 
 import build_image
 import common
 import sparse_img
-import test_utils
 from rangelib import RangeSet
+from test_utils import get_testdata_dir, ReleaseToolsTestCase
 from verity_utils import (
     CreateHashtreeInfoGenerator, HashtreeInfo,
     VerifiedBootVersion1HashtreeInfoGenerator)
 
 
-class VerifiedBootVersion1HashtreeInfoGeneratorTest(unittest.TestCase):
+class VerifiedBootVersion1HashtreeInfoGeneratorTest(ReleaseToolsTestCase):
 
   def setUp(self):
-    self.testdata_dir = test_utils.get_testdata_dir()
+    self.testdata_dir = get_testdata_dir()
 
     self.partition_size = 1024 * 1024
     self.prop_dict = {
@@ -50,9 +47,6 @@ class VerifiedBootVersion1HashtreeInfoGeneratorTest(unittest.TestCase):
     self.expected_root_hash = \
         "0b7c4565e87b1026e11fbab91c0bc29e185c847a5b44d40e6e86e461e8adf80d"
 
-  def tearDown(self):
-    common.Cleanup()
-
   def _create_simg(self, raw_data):
     output_file = common.MakeTempFile()
     raw_image = common.MakeTempFile()