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

[eskultety]

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()

Relates to: #536

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[eskultety]

/retest