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_utils.py

@@ -21,6 +21,7 @@ Utils for running unittests.
 import os
 import os.path
 import struct
+import unittest
 
 import common
 
@@ -110,3 +111,10 @@ def construct_sparse_image(chunks):
         fp.write(os.urandom(data_size))
 
   return sparse_image
+
+
+class ReleaseToolsTestCase(unittest.TestCase):
+  """A common base class for all the releasetools unittests."""
+
+  def tearDown(self):
+    common.Cleanup()