**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
self.dags = {
                key: dag
                for key, dag in self.dags.items()
                if root_dag_id != key and not (dag.is_subdag and root_dag_id == dag.parent_dag.dag_id)
            }
        if is_missing or is_expired:
            
            found_dags = self.process_file(
                filepath=correct_maybe_zipped(orm_dag.fileloc), only_if_updated=False
            )
            
            if found_dags and dag_id in [found_dag.dag_id for found_dag in found_dags]:
                return self.dags[dag_id]
            elif dag_id in self.dags:
                del self.dags[dag_id]
        return self.dags.get(dag_id)
    def _add_dag_from_db(self, dag_id: str, session: Session):
        """Add DAG to DagBag from DB"""
        from airflow.models.serialized_dag import SerializedDagModel
        row = SerializedDagModel.get(dag_id, session)
        if not row:
            raise SerializedDagNotFound(f"DAG '{dag_id}' not found in serialized_dag table")
        row.load_op_links = self.load_op_links
        dag = row.dag
        for subdag in dag.subdags:
            self.dags[subdag.dag_id] = subdag
        self.dags[dag.dag_id] = dag
        self.dags_last_fetched[dag.dag_id] = timezone.utcnow()
        self.dags_hash[dag.dag_id] = row.dag_hash
    def process_file(self, filepath, only_if_updated=True, safe_mode=True):