Set up cron job for test posterior

[njtierney]

Set up some GitHub actions to run each test posterior on a regular basis, possibly producing a tiny report to include plot outputs

[njtierney]

Consider splitting test_posterior.R into subfiles (e.g., test_posterior-geweke.R), one for each of the types of test

This is mostly so we can trigger github actions for individual test files later down the track.