[T5 Tokenizer] Fix t5 special tokens
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Fixes #5142, #8109

T5FastToeknzier and T5SlowTokenizer have different behaviors for special tokens as shown in the issue above. 
This PR fixes the slow T5 tokenizer and adds a test making sure that Fast and Slow tokenizer have the same behavior.

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 albert, bert, XLM: @LysandreJik
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 tokenizers: @mfuntowicz
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 nlp datasets: [different repo](https://github.com/huggingface/nlp)
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Comments:
Fixes #7796