selftests/coredump: add coredump server selftests

This adds extensive tests for the coredump server.

Link: https://lore.kernel.org/20250603-work-coredump-socket-protocol-v2-5-05a5f0c18ecc@kernel.org


Acked-by: Lennart Poettering <lennart@poettering.net>
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>