selftests/pidfd: add PIDFD_INFO_COREDUMP infrastructure

Add PIDFD_INFO_COREDUMP infrastructure so we can use it in tests.

Link: https://lore.kernel.org/20250516-work-coredump-socket-v8-8-664f3caf2516@kernel.org


Acked-by: Luca Boccassi <luca.boccassi@gmail.com>
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>