Fix the border-image-excess-size reftest

This was failing because the grid is now respecting border
width too. Making the selector more specific so it only
applies to the toolbars fixes this.

testsuite/reftests/border-image-excess-size.css

@@ -1,6 +1,6 @@
 @import "reset-to-defaults.css";
 
-* {
+toolbar {
   border-image: url("green-20x20.png") 5000;
   background-image: none;
   background-color: #000;