Warn on fallback in the streaming tests in cudf-polars

[Matt711]

Description

If we warn and catch it, the test will fail once we support the feature in the streaming engine. Therefore we'll keep the streaming tests up-to-date.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[TomAugspurger]

Not a problem here since it's relatively simple, but if you want to avoid repeating the assert_gpu_result_equal(...) you can put just the context manager definition in the conditional:

if should_warn:
    context = pytest.warns(UserWarning, match=...)
else:
    context = contextlib.nullcontext

with context:
    assert_gpu_result_equal(q, engine=engine, check_row_order=check_row_order)