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

[vstinner]

Convert test_concurrent_futures to a package of sub-tests.

(cherry picked from commit aa6f787)

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

[vstinner]

I backported the change to Python 3.12. There is a minor conflict in libregrtest on:

 SPLITTESTDIRS = {
     "test_asyncio",
+    "test_concurrent_futures",
 }

The backport is in conflict with PR #108401 (split multiprocessing tests) which may be merged first.

[gpshead]

this and the similar PR for test_multiprocessing_* will be a tiny merge conflict in that runtest.py list. easily resolved in one after the other goes in.