**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
if head:
                    continue
                if mod_name == '__init__':
                    self.log.warning("Found __init__.%s at root of %s", ext, filepath)
                self.log.debug("Reading %s from %s", zip_info.filename, filepath)
                if not might_contain_dag(zip_info.filename, safe_mode, current_zip_file):
                    
                    
                    if not self.has_logged:
                        self.has_logged = True
                        self.log.info(
                            "File %s:%s assumed to contain no DAGs. Skipping.", filepath, zip_info.filename
                        )
                    continue
                if mod_name in sys.modules:
                    del sys.modules[mod_name]
                try:
                    sys.path.insert(0, filepath)
                    current_module = importlib.import_module(mod_name)
                    mods.append(current_module)
                except Exception as e:  
                    self.log.exception("Failed to import: %s", filepath)
                    if self.dagbag_import_error_tracebacks:
                        self.import_errors[filepath] = traceback.format_exc(
                            limit=-self.dagbag_import_error_traceback_depth
                        )
                    else:
                        self.import_errors[filepath] = str(e)
        return mods
    def _process_modules(self, filepath, mods, file_last_changed_on_disk):