[3.11] gh-110995: Fix test_gdb check_usable_gdb() (GH-110998) (#111004)

gh-110995: Fix test_gdb check_usable_gdb() (GH-110998)

Fix detection of gdb built without Python scripting support.

* check_usable_gdb() doesn't check gdb exit code when calling
  run_gdb().
* Use shutil.which() to get the path to the gdb program.
(cherry picked from commit 920b3dfacad615c7bb9bd9a35774469f8809b453)

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