**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
):
                sd_last_updated_datetime = SerializedDagModel.get_last_updated_datetime(
                    dag_id=dag_id,
                    session=session,
                )
                if not sd_last_updated_datetime:
                    self.log.warning("Serialized DAG %s no longer exists", dag_id)
                    del self.dags[dag_id]
                    del self.dags_last_fetched[dag_id]
                    del self.dags_hash[dag_id]
                    return None
                if sd_last_updated_datetime > self.dags_last_fetched[dag_id]:
                    self._add_dag_from_db(dag_id=dag_id, session=session)
            return self.dags.get(dag_id)
        
        dag = None
        root_dag_id = dag_id
        if dag_id in self.dags:
            dag = self.dags[dag_id]
            if dag.is_subdag:
                root_dag_id = dag.parent_dag.dag_id  
        
        orm_dag = DagModel.get_current(root_dag_id, session=session)
        if not orm_dag:
            return self.dags.get(dag_id)
        
        is_missing = root_dag_id not in self.dags
        is_expired = orm_dag.last_expired and dag and dag.last_loaded < orm_dag.last_expired
        if is_expired: