**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
"""
        Given a path to a python module or zip file, this method imports
        the module and look for dag objects within it.
        """
        
        
        
        if filepath is None or not os.path.isfile(filepath):
            return []
        try:
            
            
            file_last_changed_on_disk = datetime.fromtimestamp(os.path.getmtime(filepath))
            if (
                only_if_updated
                and filepath in self.file_last_changed
                and file_last_changed_on_disk == self.file_last_changed[filepath]
            ):
                return []
        except Exception as e:  
            self.log.exception(e)
            return []
        if not zipfile.is_zipfile(filepath):
            mods = self._load_modules_from_file(filepath, safe_mode)
        else:
            mods = self._load_modules_from_zip(filepath, safe_mode)
        found_dags = self._process_modules(filepath, mods, file_last_changed_on_disk)
        self.file_last_changed[filepath] = file_last_changed_on_disk
        return found_dags
    def _load_modules_from_file(self, filepath, safe_mode):