**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
"""Save attributes about list of DAG to the DB."""
        
        from airflow.models.dag import DAG
        from airflow.models.serialized_dag import SerializedDagModel
        def _serialize_dag_capturing_errors(dag, session):
            """
            Try to serialize the dag to the DB, but make a note of any errors.
            We can't place them directly in import_errors, as this may be retried, and work the next time
            """
            if dag.is_subdag:
                return []
            try:
                
                dag_was_updated = SerializedDagModel.write_dag(
                    dag,
                    min_update_interval=settings.MIN_SERIALIZED_DAG_UPDATE_INTERVAL,
                    session=session,
                )
                if dag_was_updated:
                    self._sync_perm_for_dag(dag, session=session)