Fix broken tests

[dougiesquire]

Currently, all tests rely on AWS-hosted test data that appears to no longer exist. For now these tests have been marked as broken and are skipped in the CI, but they need to be fixed.

[anton-seaice]

For the testing of generating a cice grid from mom, this is fixed by #6

For test_corners, we could find a "Core" grid file to use as test data, or it should be able to test this using a mocked grid or a test MOM grid.