Yes. There are obviously lots of existing tests which are unit tests. But rather than attempt to move them wholesale into the new structure I've added just the new test. This is an extension of what has already happened elsewhere, e.g. `lib/iris/tests/experimental/regrid/test_regrid_area_weighted_...`

If this is going to hold up this PR at all then I suggest the proper place to have this discussion is back on the google group with @bblay, myself, and anyone else who wants to contribute. In which case I'll just adjust the test location in this PR so it can be merged ASAP.