Run wxClientDC and wxMemoryDC tests too in the graphics benchmark.

Run wxClientDC and wxMemoryDC tests too in the graphics benchmark.

These tests were not run because OnPaint() handler was called before OnCreate()
and exited the main loop before the create event could be dispatched. Fix this
by just running everything from OnPaint(), like this we're sure to do it.

Also refactor the code a little to avoid duplicating the tests.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73468 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775