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

@@ -10,6 +10,10 @@ toolbar {
   border-radius: 0;
 }
 
+grid {
+  background-color: #000;
+}
+
 grid * {
   background-color: lime;
   border-style: none;