Add models that will have negative values if treated as signed

In order to make a failure more apparent, models that use unsigned values should have a value high enough that they are negative if the client incorrectly handles them as signed.

This specifically needs to be tested for indices.

KhronosGroup/glTF-Blender-Exporter#139

Start by locating the model-generation entry point and existing index validation tests in this asset generator. Add models containing unsigned index values high enough to appear negative when incorrectly treated as signed, then run the relevant validation or importer tests. Done means index handling is covered by assets that expose signed-value errors.

Written by the indexing model from the issue text.