Use nested tempfile fixture for cleanups

There are a couple places where we appear to leak tempfile and tempdirs
because we are creating dirs/files in the tests then not deleting them
when we are done. Address this by using the nested tempfile fixture
which will nest all tempfiles and tempdirs in a tempdir that the fixture
cleans up when the test is completed.

Change-Id: I2818ed48823c544bb1be6b4e1e58f78109197bc1
diff --git a/tests/unit/test_disk_accountant.py b/tests/unit/test_disk_accountant.py
index fc4389c..e12846d 100644
--- a/tests/unit/test_disk_accountant.py
+++ b/tests/unit/test_disk_accountant.py
@@ -10,6 +10,7 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+import fixtures
 import os
 import tempfile
 import time
@@ -32,6 +33,10 @@
 
 
 class TestDiskAccountant(BaseTestCase):
+    def setUp(self):
+        super(TestDiskAccountant, self).setUp()
+        self.useFixture(fixtures.NestedTempfile())
+
     def test_disk_accountant(self):
         jobs_dir = tempfile.mkdtemp(
             dir=os.environ.get("ZUUL_TEST_ROOT", None))