[3.12] gh-116656: Fix test_capi test_py_config_isoloated_per_interpreter() (GH-116658) (#116668)

gh-116656: Fix test_capi test_py_config_isoloated_per_interpreter() (GH-116658)

Don't parse argv when setting the configuration, to avoid SystemExit if parsing argv fails.
(cherry picked from commit f6e7a6ce651b43c6e060608a4bb20685f39e9eaa)

Co-authored-by: Victor Stinner <vstinner@python.org>