tests: add tests for CLONE_PIDFD_KILL_ON_CLOSE

This adds tests for CLONE_PIDFD_KILL_ON_CLOSE:
- test that pidfd-based processes are correctly reference counted when
  created with CLONE_PIDFD_KILL_ON_CLOSE
  - test that pidfd-based processes are not killed prematurely
- that that pidfd-based processes are killed correctly when the last
  pidfd is closed

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Andy Lutomirsky <luto@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-api@vger.kernel.org
2 files changed