**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
with airflow or not
    :type include_examples: bool
    :param include_smart_sensor: whether to include the smart sensor native
        DAGs that create the smart sensor operators for whole cluster
    :type include_smart_sensor: bool
    :param read_dags_from_db: Read DAGs from DB if ``True`` is passed.
        If ``False`` DAGs are read from python files.
    :type read_dags_from_db: bool
    :param load_op_links: Should the extra operator link be loaded via plugins when
        de-serializing the DAG? This flag is set to False in Scheduler so that Extra Operator links
        are not loaded to not run User code in Scheduler.
    :type load_op_links: bool
    """
    DAGBAG_IMPORT_TIMEOUT = conf.getfloat('core', 'DAGBAG_IMPORT_TIMEOUT')
    SCHEDULER_ZOMBIE_TASK_THRESHOLD = conf.getint('scheduler', 'scheduler_zombie_task_threshold')
    def __init__(
        self,
        dag_folder: Union[str, "pathlib.Path", None] = None,
        include_examples: bool = conf.getboolean('core', 'LOAD_EXAMPLES'),
        include_smart_sensor: bool = conf.getboolean('smart_sensor', 'USE_SMART_SENSOR'),
        safe_mode: bool = conf.getboolean('core', 'DAG_DISCOVERY_SAFE_MODE'),
        read_dags_from_db: bool = False,
        store_serialized_dags: Optional[bool] = None,
        load_op_links: bool = True,
    ):
        
        from airflow.models.dag import DAG
        super().__init__()
        if store_serialized_dags:
            warnings.warn(