test: sharpen button label unit test

Using different strings for the device tree node labels and the label
property of buttons sharpens the button label unit test.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Philippe Reynes <philippe.reynes@softathome.com>
diff --git a/arch/sandbox/dts/sandbox.dtsi b/arch/sandbox/dts/sandbox.dtsi
index c76ecc0..0faad3f 100644
--- a/arch/sandbox/dts/sandbox.dtsi
+++ b/arch/sandbox/dts/sandbox.dtsi
@@ -18,14 +18,14 @@
 	buttons {
 		compatible = "gpio-keys";
 
-		summer {
+		btn1 {
 			gpios = <&gpio_a 3 0>;
-			label = "summer";
+			label = "button1";
 		};
 
-		christmas {
+		btn2 {
 			gpios = <&gpio_a 4 0>;
-			label = "christmas";
+			label = "button2";
 		};
 	};