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-style.css

@@ -1,6 +1,6 @@
 @import url("reset-to-defaults.css");
 
-GtkButton {
+button {
   background-color: red;
   border-color: lime;
   border-width: 1px;