Add comments to explain test validation (#10176)

PR to address #10151 (review)

Authors:
  - GALI PREM SAGAR (https://github.com/galipremsagar)

Approvers:
  - Bradley Dice (https://github.com/bdice)

URL: #10176

python/cudf/cudf/tests/test_list.py

@@ -593,7 +593,12 @@ def test_listcol_setitem_retain_dtype():
     df = cudf.DataFrame(
         {"a": cudf.Series([["a", "b"], []]), "b": [1, 2], "c": [123, 321]}
     )
-    df1 = df[df.b.isna()]
+    df1 = df.head(0)
+    # Performing a setitem on `b` triggers a `column.column_empty_like` call
+    # which tries to create an empty ListColumn.
     df1["b"] = df1["c"]
-    df2 = df1.drop(["c"], axis=1)
-    assert df2.a.dtype == df.a.dtype
+    # Performing a copy to trigger a copy dtype which is obtained by accessing
+    # `ListColumn.children` that would have been corrupted in previous call
+    # prior to this fix: https://github.com/rapidsai/cudf/pull/10151/
+    df2 = df1.copy()
+    assert df2["a"].dtype == df["a"].dtype