test: dm: Add tests for LED boot and activity

Add tests for LED boot and activity feature and add required property in
sandbox test DTS.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/sandbox/dts/test.dts

@@ -101,6 +101,8 @@
 			bootscr-ram-offset = /bits/ 64 <0x12345678>;
 			bootscr-flash-offset = /bits/ 64 <0>;
 			bootscr-flash-size = /bits/ 64 <0x2000>;
+			boot-led = "sandbox:green";
+			activity-led = "sandbox:red";
 		};
 	};