MDL-63466 core_message: don't reorder messages when formatting

- Removed the reordering code from helper::get_conversation_messages()
- Added unit test confirming sort param behaviour for the
api::get_conversations_messages() method.

message/classes/helper.php

@@ -202,13 +202,6 @@ class helper {
         // Store the messages.
         $arrmessages = array();
 
-        // We always view messages from oldest to newest, ensure we have it in that order.
-        $lastmessage = end($messages);
-        $firstmessage = reset($messages);
-        if ($lastmessage->timecreated < $firstmessage->timecreated) {
-            $messages = array_reverse($messages);
-        }
-
         foreach ($messages as $message) {
             // Store the message information.
             $msg = new \stdClass();