[MLv2] Implement `automatic-insights` drill (#36443)

Automatic insights drills have some unusual conditions.
This adds `metabase.lib.metadata/editable?` and checks it before
returning any drills.

On settings:
In the app, `MetabaseSettings` is a global singleton and the settings
are sometimes updated in place. In the JS testing environment several
mock settings instances can exist at once, and the global singleton does
not necessarily have the values we want for any given test.

This PR makes `metabase.lib.metadata/setting` check for `describe` and
`it` to see if we're in the testing environment, and to trust the
metadata's `settings` in that case.