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

[miss-islington]

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 920b3df)

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

• Issue: test_gdb are not skipped properly if gdb is not available (instead, an uncaught exception is raised) #110995