Use `extra_int_eq_data` in `test_matrix`

Based on !206 (merged).

• Adds a extra_matrix_data with a MatrixTestCaseMixin that adds more parameters to the scalar_int_eq ones. This is meant more for testing the direct solver, but this seemed like a nice in-between step.
• Also made test_linalg_proxy use it to generate the block/cluster indexes.