[3.12] gh-110756: Sync regrtest with main branch (#110758)

[3.12] gh-110756: Sync regrtest with main branch (#110758)

gh-110756: Sync regrtest with main branch

Copy files from main to this branch:

* Lib/test/libregrtest/*.py
* Lib/test/__init__.py
* Lib/test/__main__.py
* Lib/test/autotest.py
* Lib/test/pythoninfo.py
* Lib/test/regrtest.py
* Lib/test/test_regrtest.py

Do not modify scripts running tests such as Makefile.pre.in,
.github/workflows/build.yml or Tools/scripts/run_tests.py: do not use
--fast-ci and --slow-ci in this change.

Changes:

* SPLITTESTDIRS: don't include test_inspect.
* Add utils.process_cpu_count() using len(os.sched_getaffinity(0)).
* test_regrtest doesn't use @support.without_optimizer which doesn't
  exist in Python 3.12.
* Add support.set_sanitizer_env_var().
* Update test_faulthandler to use support.set_sanitizer_env_var().