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_udp.py

@@ -293,14 +293,14 @@ class TestUDP(VppTestCase):
                               "uri " + uri)
         if error:
             self.logger.critical(error)
-            self.assertEqual(error.find("failed"), -1)
+            self.assertNotIn("failed", error)
 
         error = self.vapi.cli("test echo client mbytes 10 appns 1 " +
                               "fifo-size 4 no-output test-bytes " +
                               "syn-timeout 2 no-return uri " + uri)
         if error:
             self.logger.critical(error)
-            self.assertEqual(error.find("failed"), -1)
+            self.assertNotIn("failed", error)
 
         # Delete inter-table routes
         ip_t01.remove_vpp_config()