Update cuDF's assert_eq import

rapidsai/cudf#16063 has updated the import
location of `assert_eq` to the public `cudf.testing.assert_eq`, this
change updates imports accordingly.

dask_cuda/tests/test_cudf_builtin_spilling.py

@@ -20,7 +20,7 @@
     get_global_manager,
     set_global_manager,
 )
-from cudf.testing._utils import assert_eq  # noqa: E402
+from cudf.testing import assert_eq  # noqa: E402
 
 if get_global_manager() is not None:
     pytest.skip(