fixup! erp5_core: Make two queries in Base_getRelatedDocumentList

My suggestion of using sorted in
nexedi/erp5!1373 (comment 129103) was wrong,
ERP5 document do not sort in deterministic order. Use a key function to make
sure we get results in same order

bt5/erp5_dms/TestTemplateItem/portal_components/test.erp5.testDms.py

@@ -2923,8 +2923,8 @@ return 1
     self.tic()
     # Related follow_up File and Embedded File
     self.assertEqual(
-      sorted([brain.getObject() for brain in getRelatedDocumentList()]),
-      sorted([sub_document_value, document_value])
+      sorted([brain.getObject() for brain in getRelatedDocumentList()], key=lambda doc: doc.getPath()),
+      sorted([sub_document_value, document_value], key=lambda doc: doc.getPath())
     )
     document_value.setFollowUpValue(None)
     self.tic()