**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
"""
        Gets the DAG out of the dictionary, and refreshes it if expired
        :param dag_id: DAG Id
        :type dag_id: str
        """
        
        from airflow.models.dag import DagModel
        if self.read_dags_from_db:
            
            from airflow.models.serialized_dag import SerializedDagModel
            if dag_id not in self.dags:
                
                self._add_dag_from_db(dag_id=dag_id, session=session)
                return self.dags.get(dag_id)
            
            
            
            
            
            
            min_serialized_dag_fetch_secs = timedelta(seconds=settings.MIN_SERIALIZED_DAG_FETCH_INTERVAL)
            if (
                dag_id in self.dags_last_fetched
                and timezone.utcnow() > self.dags_last_fetched[dag_id] + min_serialized_dag_fetch_secs