[Bug] test_unfinished_handler_on_workflow_termination fails in some situations with time skipping and update

[cretz]

Describe the bug

As part of #644 we were trying to unskip these tests. But they still fail.

Specifically, when running in Python 3.13 (and maybe others) with time skipping test server, all permutations of test_unfinished_handler_on_workflow_termination that use handler_type update, handler_waiting no-wait-all-handlers-finish, and workflow_termination_type continue-as-new fail with "workflow execution already completed".

To replicate, run:

poe test -s --log-cli-level=DEBUG --workflow-environment time-skipping -k test_unfinished_handler_on_workflow_termination

To see the failure. Leaving tests skipped for now.