The problematic scenario was having one pod in flight, one event in the list,
and then detecting a concurrent event for a second pod after the first pod is
done. The new test case covers that.
To make it work without assumptions about the implementation, the QueuedPodInfo
returned by Pop must be the one passed to AddUnschedulableIfNotPresent
after (potentially) populating UnschedulablePlugins. This is done via callback
functions which bind to the same shared variable.
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