test: Deletion request fails in an unencrypted chat and the message remains

src/receive_imf.rs

@@ -1531,6 +1531,8 @@ async fn add_parts(
     } else if let Some(rfc724_mid_list) = mime_parser.get_header(HeaderDef::ChatDelete) {
         chat_id = DC_CHAT_ID_TRASH;
         if let Some(part) = mime_parser.parts.first() {
+            // See `message::delete_msgs_ex()`, unlike edit requests, DC doesn't send unencrypted
+            // deletion requests, so there's no need to support them.
             if part.param.get_bool(Param::GuaranteeE2ee).unwrap_or(false) {
                 let mut modified_chat_ids = HashSet::new();
                 let mut msg_ids = Vec::new();