[3.11] gh-108388: Convert test_concurrent_futures to package (#108401) (#109704)

* gh-108388: Convert test_concurrent_futures to package (#108401)

Convert test_concurrent_futures to a package of sub-tests.

(cherry picked from commit aa6f787faa4bc45006da4dc2f942fb9b82c98836)

Notes on backport to 3.11:

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