I think we could generate these files on the fly. They don't even need to be grib files really, we just want to check the RoamingMagicNumber functionality.

If you want to test the grib functionality, again, we can take a conventional grib file, and add a header to the file at test-time.

My angles on this comment are two-fold:
 * try to reduce the number of files we need to run the tests (i.e. can we generate this stuff on the fly)
 * try to limit the scope of the test (i.e. do we need to use iris.load *at all*?)