[gtk] reftests: Fix background-size-zero test

[Benjamin Otte <otte src gnome org>]

commit 8846f283690c458f1b621b50ed652359289cb141
Author: Benjamin Otte <otte redhat com>
Date:   Wed Mar 27 05:19:42 2019 +0100

    reftests: Fix background-size-zero test
    
    We were drawing nothing, causing a timeout.

 testsuite/reftests/background-size-zero.css | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/testsuite/reftests/background-size-zero.css b/testsuite/reftests/background-size-zero.css
index 8e9389ba1e..8dc2d3337e 100644
--- a/testsuite/reftests/background-size-zero.css
+++ b/testsuite/reftests/background-size-zero.css
@@ -1,6 +1,7 @@
 @import "reset-to-defaults.css";
 
 .background {
+  background-color: red;
   background-image: linear-gradient(to right, blue, blue);
   background-size: 0%;
 }