Refactor Scoreswrapper CubeList code out into a fixture

> We should move this code out into a [pytest fixture](https://docs.pytest.org/en/stable/how-to/fixtures.html) within this file and use it across multiple tests. That should make our tests faster and only dependent on what they actually need.

_Originally posted by @jfrost-mo in [#2232](https://github.com/MetOffice/CSET/pull/2232/changes#r3520101940)_

Find the test file containing the Scoreswrapper CubeList setup and read pytest's fixture guidance first. Extract the setup into a fixture in that file, reuse it across the relevant tests, and run those tests to confirm they pass with only the required dependencies.

Written by the indexing model from the issue text.