**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
from airflow.models.dag import DAG  
        is_zipfile = zipfile.is_zipfile(filepath)
        top_level_dags = [o for m in mods for o in list(m.__dict__.values()) if isinstance(o, DAG)]
        found_dags = []
        for dag in top_level_dags:
            if not dag.full_filepath:
                dag.full_filepath = filepath
                if dag.fileloc != filepath and not is_zipfile:
                    dag.fileloc = filepath
            try:
                dag.is_subdag = False
                if isinstance(dag.normalized_schedule_interval, str):
                    croniter(dag.normalized_schedule_interval)
                self.bag_dag(dag=dag, root_dag=dag)
                found_dags.append(dag)
                found_dags += dag.subdags
            except (CroniterBadCronError, CroniterBadDateError, CroniterNotAlphaError) as cron_e:
                self.log.exception("Failed to bag_dag: %s", dag.full_filepath)
                self.import_errors[dag.full_filepath] = f"Invalid Cron expression: {cron_e}"
                self.file_last_changed[dag.full_filepath] = file_last_changed_on_disk
            except (
                AirflowDagCycleException,
                AirflowDagDuplicatedIdException,
                AirflowClusterPolicyViolation,
            ) as exception:
                self.log.exception("Failed to bag_dag: %s", dag.full_filepath)
                self.import_errors[dag.full_filepath] = str(exception)
                self.file_last_changed[dag.full_filepath] = file_last_changed_on_disk
        return found_dags
    def bag_dag(self, dag, root_dag):