Faster Pegasus tokenizer tests
Current test_tokenization_pegasus.py takes more than a minute to run because it uses a full size tokenizer [here](https://github.com/huggingface/transformers/blob/master/tests/test_tokenization_pegasus.py#L19)


It should use "fixtures/test_sentencepiece.model" like `tests/test_tokenization_t5.py`.
Any interest @stas00 ?<|||||>Yes, please, you can assign this to me, but most likely will be able to start on it in a few weeks when I have free time.<|||||>We can start this, but if we do we should wait for @thomwolf 's fast tokenizer PR to merge before we merge the fix.<|||||>This is unblocked, thom merged!