Add tests of expression-based sort and sort-by

We only need stable vs unstable variants for the sort-by case, since
when sorting a single column by itself there is no distinction between
stable and unstable.

python/cudf_polars/tests/expressions/test_sort.py

@@ -0,0 +1,53 @@
+# SPDX-FileCopyrightText: Copyright (c) 2024 NVIDIA CORPORATION & AFFILIATES.
+# SPDX-License-Identifier: Apache-2.0
+from __future__ import annotations
+
+import itertools
+
+import pytest
+
+import polars as pl
+
+from cudf_polars.testing.asserts import assert_gpu_result_equal
+
+
+@pytest.mark.parametrize("descending", [False, True])
+@pytest.mark.parametrize("nulls_last", [False, True])
+def test_sort_expression(descending, nulls_last):
+    ldf = pl.LazyFrame(
+        {
+            "a": [5, -1, 3, 4, None, 8, 6, 7, None],
+        }
+    )
+
+    query = ldf.select(pl.col("a").sort(descending=descending, nulls_last=nulls_last))
+    assert_gpu_result_equal(query)
+
+
+@pytest.mark.parametrize(
+    "descending", itertools.combinations_with_replacement([False, True], 3)
+)
+@pytest.mark.parametrize(
+    "nulls_last", itertools.combinations_with_replacement([False, True], 3)
+)
+@pytest.mark.parametrize("maintain_order", [False, True], ids=["unstable", "stable"])
+def test_sort_by_expression(descending, nulls_last, maintain_order):
+    ldf = pl.LazyFrame(
+        {
+            "a": [1, 2, 3, 4, 5, 6, 7, 8, 9, 10],
+            "b": [1, 2, 2, 3, 9, 5, -1, 2, -2, 16],
+            "c": ["a", "A", "b", "b", "c", "d", "A", "Z", "ä", "̈Ä"],
+        }
+    )
+
+    query = ldf.select(
+        pl.col("a").sort_by(
+            pl.col("b"),
+            pl.col("c"),
+            pl.col("b") + pl.col("a"),
+            descending=descending,
+            nulls_last=nulls_last,
+            maintain_order=maintain_order,
+        )
+    )
+    assert_gpu_result_equal(query, check_row_order=maintain_order)