leds: led-test: Provide tests for the lookup and get infrastructure

leds: led-test: Provide tests for the lookup and get infrastructure

This API allows providers to offer an specific LED to be looked-up by a
consumer.  Consumers are then able to describe the aforementioned LED
and take a reference on it.

For convenience, we're testing both sides of the API in just one test
function here.  In reality, both the provider and the consumer would be
logistically orthogonal.

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

RESULTS:
  [16:38:57] Configuring KUnit Kernel ...
  [16:38:57] 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
  [16:39:02] Starting KUnit Kernel (1/1)...
  [16:39:02] ============================================================
  Running tests with:
  $ .kunit/linux kunit.enable=1 mem=1G console=tty kunit_shutdown=halt
  [16:39:03] ===================== led (2 subtests) =====================
  [16:39:03] [PASSED] led_test_class_register
  [16:39:03] [PASSED] led_test_class_add_lookup_and_get
  [16:39:03] ======================= [PASSED] led =======================
  [16:39:03] ============================================================
  [16:39:03] Testing complete. Ran 2 tests: passed: 2
  [16:39:03] Elapsed time: 6.255s total, 0.001s configuring, 5.131s building, 1.106s running

Link: https://lore.kernel.org/r/20250501081918.3621432-3-lee@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>