Add missing templates to testdialog

This makes the template test cases not crash. The templates also
serve to test the new action widget support.

https://bugzilla.gnome.org/show_bug.cgi?id=728846

tests/testdialog.c

@@ -216,7 +216,6 @@ show_dialog_from_template (GtkWindow *parent)
                          "transient-for", parent,
                          NULL);
 
-  add_buttons (dialog);
   add_content (dialog);
 
   gtk_dialog_run (GTK_DIALOG (dialog));
@@ -238,7 +237,6 @@ show_dialog_flex_template (GtkWindow *parent)
                          "use-header-bar", use_header,
                          NULL);
 
-  add_buttons (dialog);
   add_content (dialog);
 
   gtk_dialog_run (GTK_DIALOG (dialog));