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-gradient.css

@@ -1,5 +1,4 @@
 * {
-  engine: none;
   border-image: none;
   border-radius: 0;
   border-style: solid;