rstan exposing functions is failing in CI

[seabbs]

As above this is a new issue and is causing all checks to fail. Everything works locally so the precise issue is unclear. I have hotfixed by removing skipping this test on all CI but obviously this isn't a sensible long-term solution.

See https://github.com/epinowcast/epinowcast/actions/runs/5823160125/job/15789401806