**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. 
tests/utils/test_dag_cycle.py
dag = DAG('dag', start_date=DEFAULT_DATE, default_args={'owner': 'owner1'})
        
        
        with dag:
            op1 = DummyOperator(task_id='A')
            op2 = DummyOperator(task_id='B')
            op3 = DummyOperator(task_id='C')
            op4 = DummyOperator(task_id='E')
            op5 = DummyOperator(task_id='F')
            op1.set_downstream(op2)
            op1.set_downstream(op3)
            op4.set_downstream(op1)
            op3.set_downstream(op5)
            op2.set_downstream(op5)
            op5.set_downstream(op1)
        with pytest.raises(AirflowDagCycleException):
            assert not _test_cycle(dag)