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: https://github.com/containerbuildsystem/cachi2/issues/536

Maintainers will complete the following section

• [x] Commit messages are descriptive enough
• [x] Code coverage from testing does not decrease and new code is covered
• [n/a] Docs updated (if applicable)
• [n/a] Docs links in the code are still valid (if docs were updated)

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[eskultety]

/retest