Make AsyncExecLoopExitEnum enum in the wxExecute() test public.

This fixes VC6 build as the nested class couldn't access a private enum inside
the enclosing class.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74354 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775