wxMSW could return negative client size for tiny windows with borders, this
was unexpected and shouldn't happen so explicitly ensure it does not.
Also add a unit test to check that this problem doesn't exist in other ports.
Closes#13184.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67754 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775