Flaky open_expand_close unit test for session reducer

[KeranYang]

https://github.com/numaproj/numaflow-java/actions/runs/8469748515/job/23205796860 - map-forward-message

The session reduce ServerTest failed because of receiving an AKKA dead letter.

To reproduce - run open_expand_close unit test until failure.

One observation is that if I put a 10ms sleep before invoking this test, the test became stable.