selftests/landlock: Check that coredump sockets stay unrestricted

selftests/landlock: Check that coredump sockets stay unrestricted

Even when a process is restricted with the new
LANDLOCK_ACCESS_FS_RESOLVE_UNIX right, the kernel can continue writing
its coredump to the configured coredump socket.

In the test, we create a local server and rewire the system to write
coredumps into it.  We then create a child process within a Landlock
domain where LANDLOCK_ACCESS_FS_RESOLVE_UNIX is restricted and make
the process crash.  The test uses SO_PEERCRED to check that the
connecting client process is the expected one.

Includes a fix by Mickaël Salaün for setting the EUID to 0 (see [1]).

Link[1]: https://lore.kernel.org/all/20260218.ohth8theu8Yi@digikod.net/
Suggested-by: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20260327164838.38231-11-gnoack3000@gmail.com
Signed-off-by: Mickaël Salaün <mic@digikod.net>