[3.11] gh-108388: Convert test_concurrent_futures to package (#108401)

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• Issue: Split long running test suites up into smaller sub suites to reduce our regrtest long tail on multi-core systems. #108388

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Backporting PR #108401 to 3.11 branch was challenging.

I used git cherry-pick -x, fixed conflicts, and then I "undone" manually the change by moving code again into as single Lib/test/test_concurrent_futures.py to compare changes, and I compared code before/after using meld.

I found a few minor differences, the second commit of this PR:

• AsCompletedTests: Revert test_future_times_out() => test_zero_timeout()
• Restore TODO comment
• ThreadPoolExecutorTest.test_hang_global_shutdown_lock(): add @support.requires_resource('cpu').

I also compared the new test_concurrent_futures/ directory in 3.11 and main branches. There are only minor differences. Some minor changes no backported to 3.11.

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Once this change is merged, I will be able to backport #109703 to 3.11.