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-half-pixel.css

@@ -1,6 +1,6 @@
 @import "reset-to-defaults.css";
 
-.button {
+button {
   border-top: 0.5px solid red;
   border-bottom: 0.5px solid blue;
 }