Create examples-inspired integTests

Currently, the code in `examples` submodule already serves as integration test (sort of), but pursuing other features (separate module/runnable aspirations/... see #99).

But it would be nice to have actual integration tests instead of logically residing in `atum` (core module) under src/test.

Optional:
- the tests can be distinguished by suffix (`-IntegrationSuite` perhaps) and we can even a named profile to run them