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

[vstinner]

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

Copy also changes from:

• Lib/test/support/init.py
• Lib/test/support/os_helper.py
• Lib/test/support/testresult.py
• Lib/test/support/threading_helper.py
• Lib/test/test_support.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 3.11.
• Add support.set_sanitizer_env_var().
• Update test_faulthandler to use support.set_sanitizer_env_var().
• @support.without_optimizer doesn't exist in 3.11.
• Add support.Py_DEBUG.
• regrtest.refleak: 3.11 doesn't have sys.getunicodeinternedsize.