implemented short relative date formatting for chat and added unit tests

lib/api/marianumcloud/talk/chat/get_chat_response.dart

@@ -66,7 +66,7 @@ class GetChatResponseObject {
 
   static GetChatResponseObject getDateDummy(int timestamp) {
     var elementDate = DateTime.fromMillisecondsSinceEpoch(timestamp * 1000);
-    return getTextDummy(elementDate.formatDate());
+    return getTextDummy(elementDate.formatDateRelativeShort());
   }
 
   static GetChatResponseObject getTextDummy(String text) =>