ci: fix flaky test_partial_flush_log stdout (#15285)

## Description

Attempt to address the flaky test `test_partial_flush_log`

```
FAILED tests/integration/test_integration.py::test_partial_flush_log[v0.4][py3.13] - AssertionError: STDERR: Expected [] got [b'Got response: 200 OK sent 711B in 1.59383s to http://localhost:9126/v0.4/traces\n']
```

This is just a race condition with flushing and logging, it is safe to
ignore.

## Testing

<!-- Describe your testing strategy or note what tests are included -->

## Risks

<!-- Note any risks associated with this change, or "None" if no risks
-->

## Additional Notes

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Author: brettlangdon

tests/integration/test_integration.py

@@ -758,7 +758,7 @@ def test_writer_configured_correctly_from_env_defaults_under_ddtrace_run(ddtrace
     assert status == 0, (out, err)
 
 
-@parametrize_with_all_encodings(env={"DD_TRACE_PARTIAL_FLUSH_MIN_SPANS": "2"})
+@parametrize_with_all_encodings(env={"DD_TRACE_PARTIAL_FLUSH_MIN_SPANS": "2"}, out=None)
 def test_partial_flush_log():
     import mock