Mark datasets as never-delete

Some datasets should never be deleted as it will break testing, e.g.
- `aifs-ea-an-oper-0001-mars-20p0-1979-1979-6h-v0-testing` is used to test the catalogue
-  `aifs-ea-an-oper-0001-mars-o32-2020-2021-6h-v1` used for inference integration test

I should have some way of ensuring that I don't mark these datasets as candidates for deletion.