Don't time out if process terminates when told to stop (#21103)

* Don't time out if process terminates when told to stop but response was not processed

While trying to reproduce #20822 I found a condition where the app may quit before the `app.stop` response is received but this code (which is called in integration test teardown) would sit around waiting and time out. With this change, the process exit is considered a valid response to the stop request.

* Add process exit to log for better errors in failures

This log can be turned on for debugging but is also dumped when a test fails (in certain conditions). With this in the log, it'll be clear if the timeout is because we're waiting for an event but the process quit.