Disable old migration tests

[winged]

We have a few old migration tests that are not relevant anymore. Let me elaborate:

We did quite a few refactorings of the data structure. However, for most (all) of these complex migrations, we don't have any installations anymore, and new ones will be migrated past those steps without any affected data.

Therefore, it doesn't make sense to run those slow tests in CI and local test envs anymore. They slow things down significantly, and test aspects that nobody cares about anymore.

I therefore suggest we remove those migration tests, or at least mark them with #pragma: no cover and @pytest.mark.skip(reason="nobody cares and it's slow")