Port remaining grib tests from Iris + remove circularity #88
Description
See Iris#2860 for remaining tests to be ported from Iris to Iris-grib.
This comment describes the principles wanted, that should deliver a more sensible Iris with no remaining dependencies on iris-grib (that is, even in the testing).
In addition this will be a good time to review the iris-grib codebase + weed out any remaining dependencies on private Iris api.