**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
""":return: the amount of dags contained in this dagbag"""
        return len(self.dags)
    @property
    def store_serialized_dags(self) -> bool:
        """Whether or not to read dags from DB"""
        warnings.warn(
            "The store_serialized_dags property has been deprecated. Use read_dags_from_db instead.",
            DeprecationWarning,
            stacklevel=2,
        )
        return self.read_dags_from_db
    @property
    def dag_ids(self) -> List[str]:
        """
        :return: a list of DAG IDs in this bag
        :rtype: List[unicode]
        """
        return list(self.dags.keys())
    @provide_session
    def get_dag(self, dag_id, session: Session = None):