selftests/coredump: add debug logging to coredump socket protocol tests

So it's easier to figure out bugs.

Link: https://patch.msgid.link/20251028-work-coredump-signal-v1-19-ca449b7b7aa0@kernel.org
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner