selftests/pidfd: add CLONE_NNP tests

selftests/pidfd: add CLONE_NNP tests

Add tests for the new CLONE_NNP flag:

- nnp_sets_no_new_privs: Verify a child created with CLONE_NNP has
  no_new_privs set while the parent does not.

- nnp_rejects_thread: Verify CLONE_NNP | CLONE_THREAD is rejected
  with -EINVAL since threads share credentials.

- autoreap_no_new_privs_unset: Verify a plain CLONE_AUTOREAP child
  does not get no_new_privs.

Link: https://patch.msgid.link/20260226-work-pidfs-autoreap-v5-5-d148b984a989@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>