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_fib.py

@@ -14,7 +14,7 @@ class TestFIB(VppTestCase):
 
         if error:
             self.logger.critical(error)
-        self.assertEqual(error.find("Failed"), -1)
+        self.assertNotIn("Failed", error)
 
 if __name__ == '__main__':
     unittest.main(testRunner=VppTestRunner)