regrtest: handle cross compilation and HOSTRUNNER in libregrtest, avoid Tools/scripts/run_tests.py

[vstinner]

Previously, Tools/scripts/run_tests.py was used to add Python options to the command line: -u -E -W default -bb. But regrtest is now able to do it when --fast-ci and --slow-ci options are used.

I propose moving the remaining code of Tools/scripts/run_tests.py for cross compilation and HOSTRUNNER directly in libregrtest.

Linked PRs

• gh-110152: regrtest handles cross compilation and HOSTRUNNER #110156