**Description**

function test_cycle will be collected by pytest if this function is imported in a user-defined test.
Such an error will be emitted as pytest will treat it as a test.
```
test setup failed
file /opt/conda/lib/python3.8/site-packages/airflow/utils/dag_cycle_tester.py, line 27
  def test_cycle(dag):
E       fixture 'dag' not found
>       available fixtures: cache, capfd, capfdbinary, caplog, capsys, capsysbinary, doctest_namespace, monkeypatch, pytestconfig, record_property, record_testsuite_property, record_xml_attribute, recwarn, tmp_path, tmp_path_factory, tmpdir, tmpdir_factory
>       use 'pytest --fixtures [testpath]' for help on them.

```

**Use case / motivation**
This can be simply "fixed" by renaming this function to check_cycle although the user can do it upon importing this function.
This change by airflow will reduce the amount of surprise received by the end-user, and save a large number of debugging hours. 

**Are you willing to submit a PR?**
The changeset is too small to worth the trouble for a PR. 
airflow/models/dagbag.py
"""Actual implementation of bagging a dag.
        The only purpose of this is to avoid exposing ``recursive`` in ``bag_dag()``,
        intended to only be used by the ``_bag_dag()`` implementation.
        """
        test_cycle(dag)  
        dag.resolve_template_files()
        dag.last_loaded = timezone.utcnow()
        
        settings.dag_policy(dag)
        for task in dag.tasks:
            settings.task_policy(task)
        subdags = dag.subdags
        try:
            
            
            if recursive:
                for subdag in subdags:
                    subdag.full_filepath = dag.full_filepath
                    subdag.parent_dag = dag
                    subdag.is_subdag = True
                    self._bag_dag(dag=subdag, root_dag=root_dag, recursive=False)
            prev_dag = self.dags.get(dag.dag_id)
            if prev_dag and prev_dag.full_filepath != dag.full_filepath:
                raise AirflowDagDuplicatedIdException(
                    dag_id=dag.dag_id,
                    incoming=dag.full_filepath,
                    existing=self.dags[dag.dag_id].full_filepath,
                )
            self.dags[dag.dag_id] = dag