Fix desktop_update_graphical for recent F27+ GNOME change

The default action on the reboot confirmation dialog changed
from Reboot to Cancel, so when we hit enter, we just cancel the
reboot. Tweak this to hit tab on F27+ (but not <F26, so update
tests continue to work too).

tests/desktop_update_graphical.pm

@@ -67,6 +67,9 @@ sub run {
         # logged in (but don't fail if it doesn't as we're not testing
         # that)
         if (check_screen 'gnome_reboot_confirm', 15) {
+            # on F27+, default is Cancel, earlier, default is Restart
+            my $version = lc(get_var("VERSION"));
+            send_key 'tab' if ($version eq 'rawhide' || $version > 26);
             send_key 'ret';
         }
         boot_to_login_screen;