erp5_core: Make two queries in Base_getRelatedDocumentList

For two reasons:
- In previous version, query would lose Embedded Documents, lacking a left join in follow_up_uid
- In large instances the previous version was many times slower

Also,
erp5_dms: Add a test for Base_getRelatedDocumentList

To check it works correctly with both related (follow_up)
Documents and with sub-object Embedded Files