**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/utils/dag_cycle_tester.py
"""Returns first untraversed child task, else None if all tasks traversed."""
        for adjacent_task in current_task.get_direct_relative_ids():
            if visited[adjacent_task] == CYCLE_IN_PROGRESS:
                msg = f"Cycle detected in DAG. Faulty task: {task_id}"
                raise AirflowDagCycleException(msg)
            elif visited[adjacent_task] == CYCLE_NEW:
                return adjacent_task
        return None
    for dag_task_id in dag.task_dict.keys():
        if visited[dag_task_id] == CYCLE_DONE:
            continue
        path_stack.append(dag_task_id)
        while path_stack:
            current_task_id = path_stack[-1]
            if visited[current_task_id] == CYCLE_NEW:
                visited[current_task_id] = CYCLE_IN_PROGRESS
            task = task_dict[current_task_id]
            child_to_check = _check_adjacent_tasks(current_task_id, task)
            if not child_to_check:
                visited[current_task_id] = CYCLE_DONE
                path_stack.pop()
            else:
                path_stack.append(child_to_check)