test: adjust test after jsonschema update 1

The action_params schema defines the "type" property as:

    anyOf: [{enum: [string, object, ...]}]

jsonschema v2.6.0 reported the anyOf error "matched none of the schemas"
jsonschema v3.2.0 is a bit more helpful by reporting the underlying enum
error.

I also looked at removing the anyOf (since there is only one entry), but
the anyOf makes it easier to compare our action_params schema with the
draft4 schema to make the differences very clear.

st2api/tests/unit/controllers/v1/test_packs.py

@@ -679,7 +679,10 @@ def test_packs_register_endpoint(self, mock_get_packs):
         self.assertEqual(resp.status_int, 400)
         self.assertIn(expected_msg, resp.json["faultstring"])
 
-        expected_msg = "'stringa' is not valid under any of the given schemas"
+        # with jsonschema 2.6.0, the anyOf validator errors with:
+        #   "'stringa' is not valid under any of the given schemas"
+        # with jsonschema 3.2.0, the underlying enum (anyOf->enum) gets reported instead:
+        expected_msg = "'stringa' is not one of ['array', 'boolean', 'integer', 'null', 'number', 'object', 'string']"
         self.assertEqual(resp.status_int, 400)
         self.assertIn(expected_msg, resp.json["faultstring"])