Fix `skip_special_tokens` for `Wav2Vec2CTCTokenizer._decode` (#29311)
* Fix skip_special_tokens process for Wav2Vec2CTCTokenizer._decode
* Fix skip_special_tokens for Wav2Vec2CTCTokenizer._decode
* Exclude pad_token filtering since it is used as CTC-blank token
* Add small test for skip_special_tokens
* Update decoding test for added new token