[gtk/kill-popup-windows] reftests: Fix quit-mnemonic reftest

[Matthias Clasen <matthiasc src gnome org>]

commit 493be1d92f2a7fd73eabaded6da280b0056fcf51
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed Jan 8 23:13:54 2020 -0500

    reftests: Fix quit-mnemonic reftest

 testsuite/reftests/quit-mnemonic.ui | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/testsuite/reftests/quit-mnemonic.ui b/testsuite/reftests/quit-mnemonic.ui
index 250fe4f969..5f5f4d2c3d 100644
--- a/testsuite/reftests/quit-mnemonic.ui
+++ b/testsuite/reftests/quit-mnemonic.ui
@@ -3,6 +3,8 @@
   <object class="GtkWindow" id="window1">
     <child>
       <object class="GtkGrid" id="grid1">
+        <property name="halign">center</property>
+        <property name="valign">center</property>
         <child>
           <object class="GtkLabel" id="label1">
             <property name="label" translatable="yes">Quit</property>