add radhydro problem to nightly regression tests

[BenWibking]

Describe the proposal
We currently only test hydro + optically-thin cooling in the regression test suite (https://quokka-astro.github.io/regression-testing-results/).

We should add a radhydro problem to ensure that the output and performance does not change unexpctedly.

Describe alternatives you've considered
Leave regression tests as-is.

Additional context
We have many test problems with exact solutions for radiation, but we don't currently know if there are perturbations smaller than the error thresholds.

We also don't know whether there are performance improvements/regressions over time. The regression test suite automatically tells us this, e.g.: https://quokka-astro.github.io/regression-testing-results/RandomBlast-GPU-timings.html