Add a test suite for generated code

[jfilby]

A test project would be bundled with Nexus that has YAML definitions that define ORM and web-app/service code.

To test, first run nexus gen to generate any YAML definitions into code. Unit tests within this project would then be run, which would call the generated code.

This testing should ideally run against every available DB driver, but that requires the corresponding DBs to be available. At a minimum the SQLite driver and DB should be tested with.