Cast size_t value when using it with printf() in a test.

Use %lu with a cast to avoid warnings in both 32 and 64 bit builds.

Closes #16720.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78261 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775