Tests: use self.assertNotIn().

Many tests use self.assertEqual(error.find("failed"), -1)
Use self.assertNotIn("failed", error) to provide more meaningful errors such as
AssertionError: 'Failed' not found in '' instead of 0 != -1.

Change-Id: I670acdc977b788b2cedf94cfeafc12097781463f
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>

test/test_string.py

@@ -35,7 +35,7 @@ class TestString(VppTestCase):
             error = self.vapi.cli("test string " + name)
             if error.find("failed") != -1:
                 self.logger.critical("FAILURE in the " + name + " test")
-                self.assertEqual(error.find("failed"), -1)
+                self.assertNotIn("failed", error)
 
 if __name__ == '__main__':
     unittest.main(testRunner=VppTestRunner)