**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
#
#
#
import hashlib
import importlib
import importlib.machinery
import importlib.util
import os
import sys
import textwrap
import traceback
import warnings
import zipfile
from datetime import datetime, timedelta
from typing import TYPE_CHECKING, Dict, List, NamedTuple, Optional, Union
from croniter import CroniterBadCronError, CroniterBadDateError, CroniterNotAlphaError, croniter
from sqlalchemy.exc import OperationalError
from sqlalchemy.orm import Session
from tabulate import tabulate
from airflow import settings
from airflow.configuration import conf
from airflow.exceptions import (
    AirflowClusterPolicyViolation,
    AirflowDagCycleException,
    AirflowDagDuplicatedIdException,
    SerializedDagNotFound,