Improve tokenizer tests (#13594)
* Use new method to acquire tokenizers
* Resolve TODOs.
* Style
* Fix
* Enable do_lower_case in test_tokenize_special_tokens
* Apply suggestion from code review
* Fix mask token handling
* Revert "Fix mask token handling"
This reverts commit daaa3f5291b1f71e5bc3604ca281c000000c4648.
* Fix FNet mask token tokenization
* Complete everything
* Apply suggestions from code review