[tests] fix bitsandbytes import issue

[stas00]

this test has been failing when peft library was installed since it tries to access bitsandbytes.nn

$ pip install accelerate peft bitsandbytes==0.39.0
$ RUN_SLOW="yes" pytest -sv tests/extended/test_trainer_ext.py::TestTrainerExt::test_run_seq2seq_bnb
[...]
RuntimeError: Failed to import transformers.trainer_seq2seq because of the following error (look up to see its traceback):
E           module 'bitsandbytes' has no attribute 'nn'

here is why it's happening:

1. We push transformers/tests into sys.path when running the subprocess-based tests here
2. but we have transformers/tests/bitsandbytes dir under transformers/tests
3. so when you do import bitsandbytes.nn it finds the wrong bitsandbytes dir, which is not the bnb library and it breaks

So this PR renames transformers/tests/bitsandbytes to transformers/tests/bnb which removes the conflicts and the failing test.

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[sgugger]

Thanks a lot!

[pacman100]

Thank you @stas00 for the blazing fast deep dive and the fix! 🤗