selftests/pidfd: add CLONE_PIDFD_AUTOKILL tests

selftests/pidfd: add CLONE_PIDFD_AUTOKILL tests

Add tests for CLONE_PIDFD_AUTOKILL:

- autokill_basic: Verify closing the clone3 pidfd kills the child.
- autokill_requires_pidfd: Verify AUTOKILL without CLONE_PIDFD fails.
- autokill_requires_autoreap: Verify AUTOKILL without CLONE_AUTOREAP
  fails.
- autokill_rejects_thread: Verify AUTOKILL with CLONE_THREAD fails.
- autokill_pidfd_open_no_effect: Verify only the clone3 pidfd triggers
  autokill, not pidfd_open().
- autokill_requires_cap_sys_admin: Verify AUTOKILL without CLONE_NNP
  fails with -EPERM for an unprivileged caller.
- autokill_without_nnp_with_cap: Verify AUTOKILL without CLONE_NNP
  succeeds with CAP_SYS_ADMIN.

Link: https://patch.msgid.link/20260226-work-pidfs-autoreap-v5-6-d148b984a989@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>