Handle executor events in dag.test()

[vincbeck]

If the task did not reach the task runner environment and fails, no update is made in the DB. The executor is sending "events" that are handled by the scheduler. In dag.test(), there is no scheduler, therefore we need to handle them there as well.

---

^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in newsfragments.

[o-nikolas]

Any way to add some tests so that this doesn't regress again? Does the dag.test code itself even have unit tests?

[vincbeck]

Any way to add some tests so that this doesn't regress again? Does the dag.test code itself even have unit tests?

It does but we are not testing dag.test when use_executor is True. The reason why it is not tested is it is quite hard to run an executor in pytest. Even LocalExecutor fails. And mocking the executor would result in not testing the feature. I think this should be belong to system test where an actual executor is run.