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_build_image.py b/tools/releasetools/test_build_image.py
index a2df278..6f853e8 100644
--- a/tools/releasetools/test_build_image.py
+++ b/tools/releasetools/test_build_image.py
@@ -18,15 +18,15 @@
 import math
 import os.path
 import random
-import unittest
 
 import common
 from build_image import (
     AVBCalcMinPartitionSize, BLOCK_SIZE, BuildImageError, CheckHeadroom,
     SetUpInDirAndFsConfig)
+from test_utils import ReleaseToolsTestCase
 
 
-class BuildImageTest(unittest.TestCase):
+class BuildImageTest(ReleaseToolsTestCase):
 
   # Available: 1000 blocks.
   EXT4FS_OUTPUT = (
@@ -39,9 +39,6 @@
     self._image_sizes = [BLOCK_SIZE * random.randint(51200, 524288) + offset
                          for offset in range(BLOCK_SIZE)]
 
-  def tearDown(self):
-    common.Cleanup()
-
   def test_CheckHeadroom_SizeUnderLimit(self):
     # Required headroom: 1000 blocks.
     prop_dict = {