dm: test: Add a test for the LED uclass

Add a test to confirm that we can adjust LEDs using the led_gpio driver.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index a6e8d6e..56d55ac 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -49,3 +49,5 @@
 CONFIG_RESET=y
 CONFIG_RAM=y
 CONFIG_DM_MMC=y
+CONFIG_LED=y
+CONFIG_LED_GPIO=y