Add tests for `_jax_funcify_FillDiagonal` and `_jax_funcify_PosDefMatrix`

[ricardoV94]

https://github.com/pymc-labs/roche-preclinical-hd/blob/24b710b7df0e929eba54c4397a9bdca5cd756c3d/preclinpack/jax_utils.py#L45-L58