test: dm: Test for default led naming

This modifies the existing led test to check for default led naming as
added in the previous patch.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/sandbox/dts/test.dts

@@ -559,7 +559,7 @@
 
 		default_off {
 			gpios = <&gpio_a 6 0>;
-			label = "sandbox:default_off";
+			/* label intentionally omitted */
 			default-state = "off";
 		};
 	};