Update CUDA/ROCm setup tests

[sstamenk]

Updated backend-specific setup handling and tests for CUDA/ROCm library selection. The change makes get_cuda_bnb_library_path() fail fast when the wrong override variable is used (BNB_ROCM_VERSION on CUDA or BNB_CUDA_VERSION on ROCm), improves the warning/error guidance, and refreshes the evaluator tests to use BNB_BACKEND with clearer CUDA/ROCm coverage.
Removed the test_get_rocm_bnb_library_path_rocm_override_takes_priority unit-test as it doesn't provide much utility.

This PR is a subset of the changes originally proposed in #1888

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[matthewdouglas]

Review: Update CUDA/ROCm setup tests

(Note: this review was created with the assistance of an LLM.)

Thanks for the PR! The fail-fast behavior for wrong override variables and the HIP_ENVIRONMENT → BNB_BACKEND migration are good improvements. A couple of issues to address before merging:

1. Missing path assertion in test_get_rocm_bnb_library_path_override

The diff removes the functional assertion that validates the override actually produces the correct library path:

assert get_cuda_bnb_library_path(rocm70_spec).stem == "libbitsandbytes_rocm72"

The test now only checks assert "BNB_ROCM_VERSION" in caplog.text, which verifies the warning was emitted but not that the correct library was selected. This looks like an accidental omission — please restore it.

2. test_get_cuda_bnb_library_path_override is fragile

This test sets BNB_CUDA_VERSION but doesn't clear BNB_ROCM_VERSION. With the new logic, rocm_override_value is checked first — so if BNB_ROCM_VERSION happens to be set in the environment, the test will raise RuntimeError instead of exercising the CUDA override path. Please add:

monkeypatch.delenv("BNB_ROCM_VERSION", raising=False)

(The base test test_get_cuda_bnb_library_path already does this correctly.)

Minor notes

• The dead re.sub(r"rocm\d+", ...) call in the CUDA+BNB_ROCM_VERSION error path is harmless (regex doesn't match libbitsandbytes_cuda*) but could be avoided by moving the substitution after the torch.version.cuda check.
• Consider keeping an adapted version of test_get_rocm_bnb_library_path_rocm_override_takes_priority — the "both overrides set" edge case is still reachable and worth covering, especially now that the branching logic has changed. Not blocking though.