Update cuDF's assert_eq import (#1353)

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

Authors:
  - Peter Andreas Entschev (https://github.com/pentschev)

Approvers:
  - Mads R. B. Kristensen (https://github.com/madsbk)

URL: #1353

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(