Fix test for Windows in the new wxExecute() unit test.

TestOverlappedSyncExecute() doesn't currently pass under Windows and was
supposed to not be executed there but was, in console test, as __WXMSW__ is
not defined in this case, only __WINDOWS__ is (as there is no GUI toolkit in
the console applications).

See #10258.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74404 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775