[gtk] Mark a failing reftest as xfail

[Matthias Clasen <matthiasc src gnome org>]

commit a0c6d88858f0a85fdeccd71ac7440e72165ae513
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue May 7 03:26:22 2019 +0000

    Mark a failing reftest as xfail
    
    I don't know why we expect different window types
    to render identically in the first place.

 testsuite/reftests/meson.build | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/testsuite/reftests/meson.build b/testsuite/reftests/meson.build
index 44dbc487e7..3bbfe1abdd 100644
--- a/testsuite/reftests/meson.build
+++ b/testsuite/reftests/meson.build
@@ -426,6 +426,7 @@ xfails = [
   'sizegroups-evolution-identity-page.ui',
   'textview-border-windows.ui',
   'window-show-contents-on-map.ui',
+  'toplevel-vs-popup.ui',
 ]
 
 foreach testname : testdata