Make counting of IO completion work items more precise on Windows

[kouvel]

• Follow-up to Stop counting work items from ThreadPoolTypedWorkItemQueue for ThreadPool.CompletedWorkItemCount #106854. Issue: ThreadPool.CompletedWorkItemCount double counting IO completion work items #104284.
• Before the change, the modified test case often yields 5 or 6 completed work items, due to queue-processing work items that happen to not process any user work items. After the change, it always yields 4.
• Looks like it doesn't hurt to have more-precise counting, and there was a request to backport a fix to .NET 8, where it's more necessary to fix the issue

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src/libraries/System.Threading.ThreadPool/tests/ThreadPoolTests.cs:1492

• Ensure that the test case covers scenarios where 'allWorkItemsCompleted.CheckedWait()' might timeout or fail.

allWorkItemsCompleted.CheckedWait();

[eduardo-vp]

LGTM!