TESTS: Properly set argv[0] when starting the secrets responder
    
    This was a nasty one. Because argv[0] was unsert, instead of its typical
    setting as the program path, parsing the arguments produced garbage
    which resulted in uid and gid being set to a random (?) value. As an
    effect, the secrets responder was not running as fake-root in tests and
    this triggered permission errors in libsss_secrets and only root is able
    to set arbitary secrets.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3685
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>