Clarify behaviour of test_backfill_depends_on_past (#19862)
Clarify behaviour of test_backfill_depends_on_past (#19862)

* Clarify behaviour of test_backfill_depends_on_past

It used to be that the backfill job deadlocked when a task had
"depends_on_past" set. This was because it depended on past,
non-existing and not succesful task.

You needed to set `ignore_first_depends_on_past' set in order to
avoid the deadlock. This test was quarantined but I think
the behaviour has changed.

It looks like currently Airflow behaves differently: when checking
dependencies, it actually retrieves the previous dag-run and
if there is none available it does not deadlock no matter if
depends_on_past is set. That makes much more sense and likely
was fixed during the recent changes involved in timetable
implementation.

This PR updates the tests to show that backfill will behave
properly, regardless from `ignore_first_depends_on_past`.

Fixes: #14755
nathanielvarona/airflow,mistercrunch/airflow,apache/airflow,cfei18/incubator-airflow,bolkedebruin/airflow,bolkedebruin/airflow,nathanielvarona/airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,lyft/incubator-airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,apache/airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,apache/airflow,nathanielvarona/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,apache/airflow,nathanielvarona/airflow,mistercrunch/airflow,apache/airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,bolkedebruin/airflow,lyft/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,lyft/incubator-airflow