Python Tests: Make VppTestCase unit testable.

* This allows for VppTestCase to instantiated with an empty constructor.

Change-Id: Ib71d3bfb4529e113f9b73c39de9b95f12ee9cfb4
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>

test/framework.py

@@ -1004,6 +1004,11 @@ class VppTestCase(unittest.TestCase):
 
         return rx
 
+    def runTest(self):
+        """ unittest calls runTest when TestCase is instantiated without a
+        test case.  Use case: Writing unittests against VppTestCase"""
+        pass
+
 
 def get_testcase_doc_name(test):
     return getdoc(test.__class__).splitlines()[0]