Get iris-grib testing working #3375
Description
As explained in this comment (and lots more on that stalled + rejected PR attempt).
Our Travis testing is currently failing to install a working gribapi/eccodes, so all the grib action is gaily skipped.
It's highly arguable that most of what these tests cover "ought" to be migrated to iris-grib anyway, as argued here and the other side of the fence.
But I think to be fair that is another story.