Call the superclass tearDown in VolumeTest

Currently it doesn't actually seem to make any practical difference that this
is missing, but it seems like good practice to do so anyway, to be robust
against future test case changes which might require cleanup done in the super
class.

Signed-off-by: Ian Campbell <ian.campbell@docker.com>

tests/integration/volume_test.py

@@ -18,6 +18,7 @@ class VolumeTest(DockerClientTestCase):
             except DockerException:
                 pass
         del self.tmp_volumes
+        super(VolumeTest, self).tearDown()
 
     def create_volume(self, name, driver=None, opts=None, external=None):
         if external and isinstance(external, bool):