tests: unit: package_managers: yarn: Fix mock assert called_once

Python 3.12 (not officially supported by cachi2) fails on the
    test_main.py::test_set_yarnrc_configuration

unit tests with the following error:
    AttributeError: 'called_once' is not a valid assertion. Use a spec
    for the mock if 'called_once' is meant to be an attribute.

Indeed, the proper way of handling this assertion is:
    mock_obj.assert_called_once()

Signed-off-by: Erik Skultety <eskultet@redhat.com>

tests/unit/package_managers/yarn/test_main.py

@@ -325,7 +325,7 @@ def test_set_yarnrc_configuration(
     }
 
     assert yarn_rc._data == expected_data
-    assert mock_write.called_once()
+    mock_write.assert_called_once()
 
 
 def test_verify_yarnrc_paths() -> None: