leds: Provide skeleton KUnit testing for the LEDs framework

leds: Provide skeleton KUnit testing for the LEDs framework

Apply a very basic implementation of KUnit LED testing.

More tests / use-cases will be added steadily over time.

CMD:
  tools/testing/kunit/kunit.py run --kunitconfig drivers/leds

OUTPUT:
  [15:34:19] Configuring KUnit Kernel ...
  [15:34:19] Building KUnit Kernel ...
  Populating config with:
  $ make ARCH=um O=.kunit olddefconfig
  Building with:
  $ make all compile_commands.json scripts_gdb ARCH=um O=.kunit --jobs=20
  [15:34:22] Starting KUnit Kernel (1/1)...
  [15:34:22] ============================================================
  Running tests with:
  $ .kunit/linux kunit.enable=1 mem=1G console=tty kunit_shutdown=halt
  [15:34:23] ===================== led (1 subtest) ======================
  [15:34:23] [PASSED] led_test_class_register
  [15:34:23] ======================= [PASSED] led =======================
  [15:34:23] ============================================================
  [15:34:23] Testing complete. Ran 1 tests: passed: 1
  [15:34:23] Elapsed time: 4.268s total, 0.001s configuring, 3.048s building, 1.214s running

Link: https://lore.kernel.org/r/20250424144544.1438584-1-lee@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>