reftests: Fix textview-tags test

The test was timeouting on X11.

I'm still not sure it does what it should do, but at least that
problem's gone.

testsuite/reftests/textview-tags.css

@@ -1,6 +1,6 @@
 @import "reset-to-defaults.css";
 
-GtkTextView {
+textview {
   color: yellow;
   background-color: yellow;
 }