[3.12] gh-110756: Sync regrtest with main branch

[vstinner]

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().

• Issue: regrtest: Synchronize regrtest in 3.11 and 3.12 branches with main branch #110756

[vstinner]

cc @serhiy-storchaka @sobolevn

The main drawback of this approach (copy files) is that I skipped all Changelog entries. Sorry, that's just too much work for me :-( You can look at the Changelog in the main branch for details: https://docs.python.org/dev/whatsnew/changelog.html#tests

I plan to merge this change once tests pass, only you see a good reason to not update regrtest this way.

Since I will do the same for 3.11, here are my notes on how I created this PR:

cd ../3.12
git switch -c regrtest12
/bin/cp -Rf ../main/Lib/test/libregrtest/*.py  Lib/test/libregrtest/
git add Lib/test/libregrtest/*py

/bin/cp -Rf ../main/Lib/test/pythoninfo.py  Lib/test/pythoninfo.py 
/bin/cp -Rf ../main/Lib/test/autotest.py  Lib/test/
/bin/cp -Rf ../main/Lib/test/__init__.py  Lib/test/
/bin/cp -Rf ../main/Lib/test/__main__.py  Lib/test/
/bin/cp -Rf ../main/Lib/test/regrtest.py  Lib/test/
/bin/cp -Rf ../main/Lib/test/test_regrtest.py  Lib/test/

meld Lib/test/test_faulthandler.py  ../main/Lib/test/test_faulthandler.py   # in fact at the end, both files are the same

git ci -a  # create the commit
./python -m test -j0 test_regrtest test_support 

[vstinner]

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.

We can consider backporting these changes later, but if we do that, I would prefer to do it separately.

[vstinner]

Ah, I missed some bits: PR #110779.

[bedevere-app]

GH-110781 is a backport of this pull request to the 3.11 branch.