Test models needed for texture index selection when transforming

The [KHR_texture_transform](https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_texture_transform#gltf-schema-updates) extension includes an optional `texCoord` property that can override the choice of UV index.

It would be nice to have some test model(s) that

1. check if the `texCoord` override is being respected by implementations, possibly on a number of different material inputs, and
2. help test whether the normalTexture's tangent basis is properly changed to the alternate set index.

Start with the linked KHR_texture_transform specification, focusing on the optional texCoord property and its effect on normalTexture. Create test model(s) covering the override across several material inputs and the alternate tangent basis, then verify that the assets expose both behaviors for implementations to inspect.

Written by the indexing model from the issue text.