[tests] fix bitsandbytes import issue
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](https://github.com/huggingface/transformers/blob/deff5979fee1f989d26e4946c92a5c35ce695af8/src/transformers/testing_utils.py#L1226)
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.


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