[gtk] reftests: Fix border-image-excess-size.ui test

[Benjamin Otte <otte src gnome org>]

commit d4df5211f3c8fafa599894acf69d75b4bbcf9ce4
Author: Benjamin Otte <otte redhat com>
Date:   Thu Mar 28 00:39:36 2019 +0100

    reftests: Fix border-image-excess-size.ui test
    
    The old reftests drew an opaque image, the new image is transparent.
    
    This test drew the reference image as black and the test as transparent
    black, and those are now different.

 testsuite/reftests/border-image-excess-size.css | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/testsuite/reftests/border-image-excess-size.css b/testsuite/reftests/border-image-excess-size.css
index 57ee781428..b41344cfce 100644
--- a/testsuite/reftests/border-image-excess-size.css
+++ b/testsuite/reftests/border-image-excess-size.css
@@ -10,6 +10,10 @@ toolbar {
   border-radius: 0;
 }
 
+grid {
+  background-color: #000;
+}
+
 grid * {
   background-color: lime;
   border-style: none;