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_rangelib.py b/tools/releasetools/test_rangelib.py
index e181187..1251e11 100644
--- a/tools/releasetools/test_rangelib.py
+++ b/tools/releasetools/test_rangelib.py
@@ -14,11 +14,11 @@
 # limitations under the License.
 #
 
-import unittest
-
 from rangelib import RangeSet
+from test_utils import ReleaseToolsTestCase
 
-class RangeSetTest(unittest.TestCase):
+
+class RangeSetTest(ReleaseToolsTestCase):
 
   def test_union(self):
     self.assertEqual(RangeSet("10-19 30-34").union(RangeSet("18-29")),
@@ -129,8 +129,8 @@
     self.assertEqual(
         RangeSet.parse_raw(RangeSet("0-9").to_string_raw()),
         RangeSet("0-9"))
-    self.assertEqual(RangeSet.parse_raw(
-        RangeSet("2-10 12").to_string_raw()),
+    self.assertEqual(
+        RangeSet.parse_raw(RangeSet("2-10 12").to_string_raw()),
         RangeSet("2-10 12"))
     self.assertEqual(
         RangeSet.parse_raw(RangeSet("11 2-10 12 1 0").to_string_raw()),