officejs_support_request_ui: sort support request by modification date & start date

to get stable sort and prevent "random" test failures.

bt5/erp5_officejs_support_request_ui/SkinTemplateItem/portal_skins/erp5_officejs_support_request/Folder_searchFolderSortByReferenceDescending.py

 if 'sort_on' in kw:
   del kw['sort_on']
 
-return context.searchFolder(sort_on=[('modification_date', 'descending')], **kw)
+return context.searchFolder(sort_on=[
+    ('modification_date', 'descending'),
+    # XXX to get stable test result, we also sort by start date (because modification date
+    # has a one second precision)
+    ('delivery.start_date', 'descending'),
+    ], **kw)