Handle latest testtools tearDown checks

Latest testtools checks that you only call tearDown once (it also checks
that setUp is only called once). Zuul's test framework was calling
tearDown from a cleanup which resulted in tearDown being called twice.
Once by the test framework and once by zuul's testclasses cleanups.

Just remove the explicit tearDown call in the cleanups and let the
larger python tooling call tearDown for us.

Change-Id: Id8eaa9da5153fdfe9b9f852d2c57838736cb7040
diff --git a/tests/base.py b/tests/base.py
index 179f4f4..753bc5e 100755
--- a/tests/base.py
+++ b/tests/base.py
@@ -976,7 +976,6 @@
         threads = threading.enumerate()
         if len(threads) > 1:
             self.log.error("More than one thread is running: %s" % threads)
-        super(ZuulTestCase, self).tearDown()
 
     def init_repo(self, project):
         parts = project.split('/')