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_mtu.py

@@ -33,6 +33,10 @@ class TestMTU(VppTestCase):
         cls.create_pg_interfaces(range(2))
         cls.interfaces = list(cls.pg_interfaces)
 
+    @classmethod
+    def tearDownClass(cls):
+        super(TestMTU, cls).tearDownClass()
+
     def setUp(self):
         super(TestMTU, self).setUp()
         for i in self.interfaces: