[tests] guard peft imports in test_lora_loader_utils (#14434)
The Hub staging-tests job runs `pytest -m "is_staging_test" tests`, which
imports every test module at collection time, but that job doesn't install
peft. The top-level `from peft import ...` added in #14385 therefore breaks
collection with exit code 2 on every PR.
Guard the imports behind `is_peft_available()` (as `tests/lora/utils.py`
already does) and mark the one test that needs them with
`require_peft_backend`.
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
