Fix LayoutXLMProcessorTest
# What does this PR do?

Fix `LayoutXLMProcessorTest` test failure found in

https://github.com/huggingface/transformers/runs/6663596212?check_suite_focus=true

```
E               ValueError: Calling LayoutXLMTokenizerFast.from_pretrained() with the path to a single file or url is not supported for this tokenizer. Use a model identifier or the path to a directory instead.
```

I just use a tiny model's hub name to fix it.
_The documentation is not available anymore as the PR was closed or merged._