[tests] add require_bitsandbytes marker

[faaany]

What does this PR do?

The below 2 tests fails because I haven't installed bitsandbytes in my environment:

FAILED tests/models/persimmon/test_modeling_persimmon.py::PersimmonIntegrationTest::test_model_8b_chat_greedy_generation - ImportError: Using `bitsandbytes` 8-bit quantization requires Accelerate: `pip install accelerate` and the latest version of bitsandbytes: `pip install -i https://pypi.org/simple/ bitsandbytes`
FAILED tests/models/persimmon/test_modeling_persimmon.py::PersimmonIntegrationTest::test_model_8b_chat_logits - ImportError: Using `bitsandbytes` 8-bit quantization requires Accelerate: `pip install accelerate` and the latest version of bitsandbytes: `pip install -i https://pypi.org/simple/ bitsandbytes`

But these 2 tests should be skipped instead of failed. This PR fixes this issue by adding the require_bitsandbytes marker.

Who can review?

@ArthurZucker

[amyeroberts]

Thanks!