Add more extensive testing for ABI output and function selectors

[ggiraldez]

From this discussion: #1499 (comment)

We want to add more extensive testing of computed ABI and function selectors. We can add snapshot-like tests similar to existing ones for the parser and bindings. Another alternative especially for ABI is to verify against the available sourcify metadata information. The main con of the approach is that it won't run for all PRs.

[kuzdogan]

Note that we also export the function signatures and selectors within the DB export: https://docs.sourcify.dev/docs/repository/sourcify-database/

Alternatively you can use the API endpoints too https://docs.sourcify.dev/docs/api/#4byte-signature-service-api-documentation

[ggiraldez]

Ah, I didn't notice that we have the selectors information as well. Even better then. Thanks for the info!