add union test

[cataggar]

This allows a tests.rs file for a generated service.

It now just adds a union test and adds anyhow as a dev-dependency for services.
This PR does not add the tests to CI. That would be next.

[heaths]

Seems fine, but just to double check my understanding of why: effectively, it's a file in an otherwise generated location, so you want to move it, regen, then move it back (as well as reference it in lib.rs),yeah?