[stable-2.6] Fix rabbitmq_plugin test.

These changes were part of a feature pull request, which is why they were not backported:

https://github.com/ansible/ansible/pull/48599
(cherry picked from commit cd9b364da8)

Co-authored-by: Matt Clay <matt@mystile.com>

test/integration/targets/rabbitmq_plugin/tasks/tests.yml

@@ -6,6 +6,7 @@
     rabbitmq_plugin:
       name: "{{ plugin_name }}"
       state: enabled
+      new_only: True
     register: result
 
   - name: Get rabbitmq-plugins output
@@ -25,6 +26,7 @@
     rabbitmq_plugin:
       name: "{{ plugin_name }}"
       state: enabled
+      new_only: True
     register: result
 
   - name: Check idempotency