Separate integration tests out of core unit tests

[karlvr]

Make an integration tests package, and then individual generators can use that package to reuse openapi specification files, and out of the box do compilation tests on each of them?

The integration tests suffer a little from not generating real source code and using a compiler to test that it's good. Maybe the base integration tests can use a simple TypeScript generator so we can do that. Instead of just testing expectations of the model.