Try to move the end-to-end test to be an actual external-API test

[marsella]

It doesn't really make sense that our end to end test full_protocol_execution_works poses as a unit test. It uses the public API of the library and should be an obvious and external test, since that's what it is. The only possible reason is that we're accessing some internal types to do validation; I'm not sure if this is the case.

• Make an external test module per Rust's standard guidelines
• Try to move the full protocol test to the external module
• If there are any blockers / good reasons to keep it internal as a unit test, document them very clearly at the test