Tests Cleanup: Fix missing calls to setUpClass/tearDownClass.

Continuation/Part 2 of https://gerrit.fd.io/r/#/c/17092/

Change-Id: Id0122d84eaf2c05d29e5be63a594d5e528ee7c9a
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>

test/test_l2xc.py

@@ -76,6 +76,10 @@ class TestL2xc(VppTestCase):
             super(TestL2xc, cls).tearDownClass()
             raise
 
+    @classmethod
+    def tearDownClass(cls):
+        super(TestL2xc, cls).tearDownClass()
+
     def setUp(self):
         super(TestL2xc, self).setUp()
         self.reset_packet_infos()