tests: Move testtools to 3rd party libraries section

TrivialFix

Change-Id: I78293da953dc5d0cde9c66ea7f7c06348a5315c2
diff --git a/neutron/tests/tempest/api/test_trunk_negative.py b/neutron/tests/tempest/api/test_trunk_negative.py
index a5ed4d5..b26dd75 100644
--- a/neutron/tests/tempest/api/test_trunk_negative.py
+++ b/neutron/tests/tempest/api/test_trunk_negative.py
@@ -12,11 +12,10 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import testtools
-
 from oslo_utils import uuidutils
 from tempest.lib import exceptions as lib_exc
 from tempest import test
+import testtools
 
 from neutron.tests.tempest.api import test_trunk