Do not have Django wipe out our test database during Python tests.

In tools/test-backend, we build the test database before calling
"manage.py test, so there is no reason for Django to wipe it
clean.

(imported from commit 7b09e9cc1a9259900ec1cc4f9ebbcacab1efb265)

zephyr/tests.py

@@ -2983,9 +2983,7 @@ class Runner(DjangoTestSuiteRunner):
     def run_tests(self, test_labels, extra_tests=None, **kwargs):
         self.setup_test_environment()
         suite = self.build_suite(test_labels, extra_tests)
-        old_config = self.setup_databases()
         self.run_suite(suite)
-        self.teardown_databases(old_config)
         self.teardown_test_environment()
         print 'DONE!'
         print