test driver: avoid using mutation on bytes (#501)

crates/moonbuild/template/test_driver/wasm_args.mbt

@@ -13,19 +13,21 @@ fn moonbit_test_driver_internal_string_read_char(handle : MoonbitTestDriverInter
 
 fn moonbit_test_driver_internal_finish_read_string(handle : MoonbitTestDriverInternalStringReadHandle) = "__moonbit_fs_unstable" "finish_read_string"
 
+fn fixedarray_to_bytes(arr : FixedArray[Byte]) -> Bytes = "%identity"
+
 fn moonbit_test_driver_internal_string_from_extern(e : MoonbitTestDriverInternalExternString) -> String {
-  let buf = Bytes::new(512)
+  let buf = FixedArray::make(512, Byte::default())
   let mut len = 0
   let handle = moonbit_test_driver_internal_begin_read_string(e)
   while true {
     let ch = moonbit_test_driver_internal_string_read_char(handle)
     if ch == -1 {
       break
     } else {
-      let written_bytes_len = buf.set_utf16_char(len, Char::from_int(ch))
+      let written_bytes_len = buf.set_utf16le_char(len, Char::from_int(ch))
       len += written_bytes_len
     }
   }
   moonbit_test_driver_internal_finish_read_string(handle)
-  buf.to_unchecked_string(offset = 0, length = len).to_string()
+  fixedarray_to_bytes(buf).to_unchecked_string(offset = 0, length = len).to_string()
 }