Add tests for material handling on a model with MatML link

Test the behavior of the materials when the model has a link to a MatML file. The expected behavior is that materials cannot be added.

To check: should editing and deleting also be unavailable?

Start by locating the existing material-handling tests and the model setup used for MatML links. Add coverage for the stated case that materials cannot be added, then resolve whether editing and deleting should also be unavailable before defining the complete expected behavior.

Written by the indexing model from the issue text.