[placement] add coverage for update of standard resource class

In microversion <= 1.6 it is possible to update a resource class to have
a new name. This is only possible for custom resource classes, but there
was no test to show that the proper 400 was returned when attempting to
change a standard class.

This adds one test to cover it, and in the process changes the name of
a related test to distinguish it from the new one.

Change-Id: I6c665db68de91c0de91c16f7e622f0c7e2f28bc2
Closes-Bug: #1723123

nova/tests/functional/api/openstack/placement/gabbits/resource-classes.yaml

@@ -121,7 +121,7 @@ tests:
       $.resource_classes[10].name: $ENVIRON['CUSTOM_RES_CLASS']
       $.resource_classes[10].links[?rel = "self"].href: /resource_classes/$ENVIRON['CUSTOM_RES_CLASS']
 
-- name: update standard resource class
+- name: update standard resource class bad json
   PUT: /resource_classes/VCPU
   request_headers:
     content-type: application/json
@@ -134,6 +134,20 @@ tests:
   response_json_paths:
       $.errors[0].title: Bad Request
 
+- name: update standard resource class to custom
+  desc: standard classes cannot be updated
+  PUT: /resource_classes/VCPU
+  request_headers:
+    content-type: application/json
+    OpenStack-API-Version: placement 1.6
+  data:
+      name: $ENVIRON['CUSTOM_RES_CLASS']
+  status: 400
+  response_strings:
+      - Cannot update standard resource class VCPU
+  response_json_paths:
+      $.errors[0].title: Bad Request
+
 - name: update custom resource class to standard resource class name
   PUT: /resource_classes/$ENVIRON['CUSTOM_RES_CLASS']
   request_headers: