Document tips and tricks when working on LocalTest

LocalTest is powerful, but it's not obvious (to app developers especially) what you can do with it. We could/should extend [the documentation](https://docs.altinn.studio/app/testing/local/) to describe:

- [ ] How to temporarily change profile options, like the user language (if you don't have [showLanguageSelector](https://docs.altinn.studio/app/development/ux/texts/translation/#activate-translation) on for your app)
- [x] How to add custom claims
- [x] How to add/change roles/parties/orgs etc (i.e. all the content in `TestData`)
- [x] (Regression) testing with older frontend versions
- https://docs.altinn.studio/app/testing/local/debug/#using-other-frontend-versions
- [ ] Changing the storage path to mount to a local folder, inspecting the XML containing instance data, etc.

Start with the LocalTest documentation at docs.altinn.studio/app/testing/local/ and review the existing guidance, including the linked frontend-version section. Research how profile options can be changed temporarily and how to change the storage path and inspect instance XML; done means both unchecked topics are documented with clear, verified steps.

Written by the indexing model from the issue text.