Fix wxFindWindowAtPoint() unit test to pass under GTK.

Fix wxFindWindowAtPoint() unit test to pass under GTK.

We need to ensure that all windows are realized before querying their
positions on screen, so add an extra wxYield().

Also adjust the tests slightly as the windows are now all created in the
beginning of the function.

Finally, use Destroy() instead of wxDELETE() for windows.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72608 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775