Python: Mark flaky test with xfail until we can fix it. (#4467)

### Motivation and Context

The `test_can_execute_stepwise_plan` integration test succeeds sometimes
and fails others. Marking it with `xfail` until we can root-cause the
issue. It's blocking PRs from completing successfully.

Underlying issue is #4466.

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### Description

See above.

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### Contribution Checklist

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- [X] The code builds clean without any errors or warnings
- [X] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [X] All unit tests pass, and I have added new tests where possible
- [X] I didn't break anyone 😄

Co-authored-by: Evan Mattson <[email protected]>

python/tests/integration/planning/stepwise_planner/test_stepwise_planner.py

@@ -150,6 +150,9 @@ async def test_can_create_stepwise_plan(
     ],
 )
 @pytest.mark.asyncio
+@pytest.mark.xfail(
+    reason="Test is known to occasionally produce unexpected results.",
+)
 async def test_can_execute_stepwise_plan(
     get_aoai_config,
     get_bing_config,