reftests: fix background-size reftest

We have to be careful and not set the background on the labels
themselves.

https://bugzilla.gnome.org/show_bug.cgi?id=670743

tests/reftests/background-size.css

@@ -1,6 +1,6 @@
 @import "reset-to-defaults.css";
 
-* {
+.button {
   background-image: url("border-image-balls.png");
   color: black;
   text-shadow: 1px 1px white;