Expose testing fixtures for uniffi_bindgen generators that live outside this repository

[tarikeshaq]

This is a follow-up for #1150 once it lands and the code associated with it is landed.

I'll mark this as discuss for now, it will probably require another ADR and some code changes to publish crates and define how they're expected to be consumed.

┆Issue is synchronized with this Jira Task ┆Issue Number: UNIFFI-128