Radiation port lacks a regression test

[brianhenn]

Since the deletion of the serialized fv3gfs-fortran data that supported the regression test of the python port of the radiation code, the port lacks a functioning regression test. That test was brittle and had high overhead because it depended on reading serialbox-generated data, so replacing it with a python-only test using the current code makes sense anyway. One challenge will be how to make a regression test for the stochastic MCICA radiation scheme -- the prior test actually serialized all of the random numbers, which might be difficult to reimplement. Instead we could port a deterministic (non-MC) option that already exists in the Fortran version and test that, or we could just test the deterministic clear sky part.