posix_spawn_file_actions_addfchdir tests: Enhance test. * tests/test-posix_spawn5.c: Include findprog.h. (test): New function, extracted from main. (main): Invoke it. Also, invoke it with a program name such as "bin/pwd". * modules/posix_spawn_file_actions_addfchdir-tests (Depends-on): Add findprog.