fix flaky test_unretrieved_future_exception_server_crash

[graingert]

Changes

Fixes #.

Checklist

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• You've added tests (in tests/) added which would fail without your patch
• You've updated the documentation (in docs/, in case of behavior changes or new
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Updating the changelog

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If, say, your patch fixes issue #123, the entry should look like this:

- Fix big bad boo-boo in task groups
  (`#123 <https://github.com/agronholm/anyio/issues/123>`_; PR by @yourgithubaccount)

If there's no issue linked, just link to your pull request instead by updating the
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[agronholm]

Closing in favor of the fix in #853.