Add tests for airflow dags
This PR adds the capability to test Airflow DAG instances via pytest, as described in
[SPIKE] Investigate Different CI Checks.
What changes with this PR
A new test suite for DAG integrity testing has been added.
Integrity testing
The tests/dags/test_dag_integrity.py suite performs validation of all DAG modules in the repo; it follows a recommended validation practice described in https://www.astronomer.io/guides/testing-airflow#dag-validation-testing:
ensure that your DAG objects are defined correctly, acyclic, and free from import errors.
The suite can be triggered via: make test_dags.
Note: dags/ima.py contains side effects that cause the test to (correctly) fail. We should get rid of in e.g. T292740. We could xfails() or skip() the suite before merging.