T8518: add unit tests for vyos_vlan module (#471)

Adds 7 unit tests for vyos_vlan covering present, present_no_change,
absent, absent_no_change, aggregate, purge, and address scenarios.
Fixture data moved to dedicated .cfg files under fixtures/.

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Co-authored-by: omnom62 <75066712+omnom62@users.noreply.github.com>
Co-authored-by: Daniil Baturin <daniil@baturin.org>