reftests: Fix up for easy CSS changes

This is mostly search and replace ala
  GtkButton => button
  GtkWindow => window
  .button => button
or removing style properties that aren't used anymore like
  -GtkButton-default-border: 0

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

@@ -1,5 +1,6 @@
+@import "reset-to-defaults.css";
+
 * {
-  engine: none;
   border-image: url("green-20x20.png") 5000;
   background-image: none;
   background-color: #000;
@@ -9,7 +10,7 @@
   border-radius: 0;
 }
 
-GtkGrid * {
+grid * {
   background-color: lime;
   border-style: none;
   border-width: 0;