tests/liveupdate: add in-kernel liveupdate test

Introduce an in-kernel test module to validate the core logic of the Live
Update Orchestrator's File-Lifecycle-Bound feature.  This provides a
low-level, controlled environment to test FLB registration and callback
invocation without requiring userspace interaction or actual kexec
reboots.

The test is enabled by the CONFIG_LIVEUPDATE_TEST Kconfig option.

Link: https://lkml.kernel.org/r/20251218155752.3045808-6-pasha.tatashin@soleen.com
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Alexander Graf <graf@amazon.com>
Cc: David Gow <davidgow@google.com>
Cc: David Matlack <dmatlack@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kees Cook <kees@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Pratyush Yadav <pratyush@kernel.org>
Cc: Samiullah Khawaja <skhawaja@google.com>
Cc: Tamir Duberstein <tamird@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/Kconfig.debug

@@ -2825,6 +2825,29 @@ config LINEAR_RANGES_TEST
 
 	  If unsure, say N.
 
+config LIVEUPDATE_TEST
+	bool "Live Update Kernel Test"
+	default n
+	depends on LIVEUPDATE
+	help
+	  Enable a built-in kernel test module for the Live Update
+	  Orchestrator.
+
+	  This module validates the File-Lifecycle-Bound subsystem by
+	  registering a set of mock FLB objects with any real file handlers
+	  that support live update (such as the memfd handler).
+
+	  When live update operations are performed, this test module will
+	  output messages to the kernel log (dmesg), confirming that its
+	  registration and various callback functions (preserve, retrieve,
+	  finish, etc.) are being invoked correctly.
+
+	  This is a debugging and regression testing tool for developers
+	  working on the Live Update subsystem. It should not be enabled in
+	  production kernels.
+
+	  If unsure, say N
+
 config CMDLINE_KUNIT_TEST
 	tristate "KUnit test for cmdline API" if !KUNIT_ALL_TESTS
 	depends on KUNIT