[FEA] Change cudf_polars testing assertion function to require an `engine` argument.

[mroeschke]

#22212 refactored the cudf_polars test suite to eventually parametrize over pytest fixtures of the engines we want to support (SPMD, Dask, Ray). We mostly pass this engine fixture to our assertion function in python/cudf_polars/cudf_polars/testing/asserts.py, but

• assert_gpu_result_equal and assert_sink_result_equal provides the "default" if not specified

cudf/python/cudf_polars/cudf_polars/testing/asserts.py

Line 358 in d70bda4

engine = engine or get_default_engine(executor)

cudf/python/cudf_polars/cudf_polars/testing/asserts.py

Line 100 in d70bda4

engine = engine or get_default_engine(executor)

• assert_ir_translation_raises, assert_collect_raises, and assert_sink_ir_translation_raises don't provide a way to override a hardcoded engine

cudf/python/cudf_polars/cudf_polars/testing/asserts.py

Line 164 in d70bda4

translator = Translator(q._ldf.visit(), GPUEngine())

cudf/python/cudf_polars/cudf_polars/testing/asserts.py

Line 293 in d70bda4

engine = GPUEngine(raise_on_fail=True)

cudf/python/cudf_polars/cudf_polars/testing/asserts.py

Line 422 in d70bda4

engine="gpu",

therefore we are potentially not testing the pytest engine fixture in our test.

We should change all these assertion function to require an engine argument to avoid this.

(This was discovered in #22281 when changing the default to rapidsmpf change the behavior of some of these assertions)

cc @madsbk when you're adding Ray testing

[madsbk]

Once #22348 is merged, I have a follow-up ready (#22359) that adds RayEngine to the test matrix.
We need #22348 first, though; otherwise the overhead is huge, hours instead of minutes.