Move the existing USDZ export test from C++ to Python for better
validation. The resulting file is now run through the `usdchecker`
system and we also check the contents of the resulting archive for the
expected texture file. This helped uncover a pathing issue in the
resulting archive where the 'textures' directory was misnamed on win32;
though it was still functional.
Pull Request: https://projects.blender.org/blender/blender/pulls/144176
5a5f768938