Deprecate 'airflow dags test' and merge the use case into 'airflow dags backfill' and 'airflow dags trigger'

### Description Currently (in versions up to 2.1.4), `airflow dags test <dag_id> <execution_date>` creates a *backfill* run at the specified datetime. This, however, applies regardless of whether the DAG can actually have a _logically_ automated backfill at that specific datetime or not. One example of this logically confusing behaviour is shown in #18473. A DAG with `schedule_interval=None` should logically have no backfill runs ever, but the `test` command would still happily create a backfill run at that datetime. With the introduction of custom timetables in AIP-39, the DAG scheduling logic went through some extensive refactoring to conform more closely to the DAG's schedule/timetable specification. This means that a backfill run can no longer be created at will. The 2.2 release will contain a hack to keep the current behaviour of "free" backfill run creation via `test` (#18742), but I would prefer this to be a temporary measure and be removed once we have a better solution. The root cause to this issue is, IMO, `airflow dags test` has very poor semantic as currently designed. It is entirely non-obvious it is creating *backfill* runs (and a subsequent `airflow dags backfill` call would therefore skip the specific datetime if and only if it lies on the logical schedule), nor why a backfill can happen without considering the schedule (it is the only way to do that in Airflow AFAIK). And the name `test` itself is somewhat a misnomer—why is creating a backfill run a test in the first place? ### Use case/motivation From what I can tell, the currently primary use case to `airflow dags test` is to check whether a DAG implements the tasks reasonably before it's activated. For this particular use case, the user does not actually care what kind of run is used, so a manual run would do. But we should also create a migration path for those relying on `airflow dags test` to create a backfill run, since the implied side effect of saving a backfill run later on is also somewha…