MDL-79247 forum: Make discussion list ordering deterministic

Note that normally this doesn't matter much, but there are situations
when we want the discussion list ordering fully consistent /
deterministic.

Specifically, when discussions (or forum posts )are created in the
same second, or when the discussion titles are repeated, or 2
discussions have the same number of votes... (any criteria in general),
in the context of testing, we don't want the order
to be non-consistent, so we need to provide an extra sorting
criterion to make it fully deterministic.

So, in this case, we are adding a sort by discussion.id <<DIRECTION>>
that is an unique value, primary key... so cheap to calculate and,
that way, when the 1st sorting column has repeated values, the id
will decide.

mod/forum/classes/local/vaults/discussion_list.php

@@ -347,7 +347,7 @@ private function get_sort_order(?int $sortmethod, bool $includefavourites = true
             $favouritesort .= ", {$favalias}.itemtype DESC";
         }
 
-        return "{$alias}.pinned DESC $favouritesort , {$keyfield} {$direction}";
+        return "{$alias}.pinned DESC $favouritesort , {$keyfield} {$direction}, {$alias}.id {$direction}";
     }
 
     /**