Use wxWindow::Refresh() instead of artificial wxPaintEvent in the test.

Use wxWindow::Refresh() instead of artificial wxPaintEvent in the test.

This fixes memory leaks under wxMSW: as the test code didn't (and couldn't)
call wxPaintDCImpl::EndPaint(), there was a leak for each wxPaintEvent
generated in it since the changes to wxDC caching in r72938.

It's also preferable because it uses public API instead of feeding events to
the window which is not documented to work.

Unfortunately even using Refresh() still doesn't allow the test to work under
wxOSX.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73929 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775