Add a TextView reftests checking border-windows

TextView border windows are internal windows used to draw on the gutter
of the textview (e.g. line numbers). The test uses the gmodule hook to
programmatically draw on the border-windows at each side of the textview
and compares the result with 3x3 grid of labels.

testsuite/reftests/textview-border-windows.css

@@ -0,0 +1,21 @@
+@import "reset-to-defaults.css";
+
+.top,
+#label_n {
+  background-color: blue;
+}
+
+.left,
+#label_w {
+  background-color: green;
+}
+
+.right,
+#label_e {
+  background-color: yellow;
+}
+
+.bottom,
+#label_s {
+  background-color: red;
+}