tests: do not mark changed the preparation task.

Author: steenzout

tests/test.yml

@@ -9,6 +9,7 @@
     - raw: sudo apt-get update -qq && sudo apt-get install -qq python2.7
       when: playbook_python2 is defined and
             playbook_python2
+      changed_when: false
 
 - name: setup stretch
   hosts: "{{ vagrant_box }}"