Add real-world E2E tests

[sheinbergon]

Things to check first

• I have searched the existing issues and didn't find my feature already requested there

Feature description

While the current test coverage is high, the test are mostly generation centric.
Code generation will always be as correct as much the programmer's understanding of SQLAlchemy spec.

However, we are lacking the ability to evaluate the generated code against bonafide databae instances in several simple use cases

Use case

We are encountering occasional regressions due the lack of such tests

[agronholm]

Doing this can be worth the trouble but only if you can show that they can actually catch bugs that our current testing methods do not.

[sheinbergon]

Adding native enum support would have benefitted at least twice from having such tests