Add a unit test for wxImage::Scale() method.

Add a unit test for wxImage::Scale() method.

Check that resizing the test horse image produces the same results in the
future as it does now, by saving the current results in files and verifying
that images resized directly and loaded from these files are the same.

See #15281.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74318 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775