selftests/coredump: cleanup coredump tests

Make the selftests we added this cycle easier to read.

Link: https://lore.kernel.org/20250603-work-coredump-socket-protocol-v2-3-05a5f0c18ecc@kernel.org
Acked-by: Lennart Poettering <lennart@poettering.net>
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner