Resultify get_chat_id_by_grpid

I want to avoid having to be able to represent a chat_id of 0 in order
to more nicely turn chat_id into a ChatId newtype.

src/dc_tools.rs

@@ -653,7 +653,6 @@ mod tests {
         );
     }
 
-    #[test]
     #[test]
     fn test_dc_extract_grpid_from_rfc724_mid() {
         // Should return None if we pass invalid mid