Add proper tests

[floriankrb]

The current tests do not pass in the ci. They need to be run manually and this is not a good practice.

Points to solve to do this:

• The granularity of these tests may not be correct. They need a new dataset to register it in the catalogue. To have this anemoi-datasets is needed. They are not unit testing, but closer to integration testing.
• They actually use the real anemoi.ecmwf.int production server, we would need a dedicated test server. Or change the granularity.
• They need admin credential to the server, which makes it difficult for contributors to run the tests locally before submitting a PR.

Somehow, this is related/would be addressed with having proper integration tests for the whole anemoi-* packages.