[pegasus] Faster tokenizer tests
This PR implements #7354

* The suggested `fixtures/test_sentencepiece.model` couldn't be used since it has wrong special token ids: we need 
   1. no bos
   2. eos_id is 1
   3. unk_id is 2

   added a script that builds a custom tokenizer model: `test_sentencepiece_no_bos.model`. Had to figure out how to match `"google/pegasus-large"` spm file. See the build script for nuances.
* switched pegasus common tests to use the newly added `test_sentencepiece_no_bos.model` - 2 custom tests still use the large tokenizer - remained untouched

And a few extra tweaks I made while sorting this PR out:

* removed `get_vocab` in `tokenization_pegasus.py` as it's identical to superclass's one
* a few minor prose edits in related files
* expanded `testing_utils.py`' s `get_tests_dir` to accept an optional `append_path` arg to remove clutter from tests. will probably rename it in the future to something else, works for now.



Fixes #7354

@sshleifer, @LysandreJik 