Add a test of precisely sized status bar fields.

Modify the status bar sample to use two fields of just the right size for
their contents to confirm that this doesn't work correctly under at least MSW
currently (see #10696).

Notice that we need two fields because the behaviour/problem is different for
the last field and all the other ones.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61991 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775