reftests: Add a hack to make toplevels work in various WMs

reftests: Add a hack to make toplevels work in various WMs

If we have a toplevel, and not a popup window, do wait an additional
0.5s to give the WM/server enough time to actually create the window.

This is a hack and there should be a better solution. But it works.

Please use POPUP windows for tests unless the test must use toplevel
windows.