selftests/exec: execveat: Improve debug reporting

Children processes were reporting their status, duplicating the
parent's. Remove that, and add some additional details about the test
execution.

Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Link: https://lore.kernel.org/r/20240313185606.work.073-kees@kernel.org


Signed-off-by: Kees Cook <keescook@chromium.org>