Ensure that size in the minor direction of box sizer doesn't exceed the total.

Ensure that size in the minor direction of box sizer doesn't exceed the total.

This is similar to the previous commit but for the transversal direction of a
box sizer: we could give an item size larger than the size of the sizer itself
making only part of its window visible (and thus potentially making the window
unusable e.g. because the scrollbar wasn't visible at all).

Fix this by always restricting the item size in the minor direction to the
total size available and add a unit test which failed previously and passes
now.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63705 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775