Improve truncation_side (#14947)
* Enabling `truncation_side` for Slow and Fast tokenizer.
Co-Authored-by: Niels Rogge <48327001+NielsRogge@users.noreply.github.com>
* Disable failing tests.
* Layout xlm.
* assert -> assertEqual.
Co-authored-by: Niels Rogge <48327001+NielsRogge@users.noreply.github.com>