T8518: add unit tests for vyos_vlan module
- Fix copyright header to use VyOS Networks attribution
- Add 'Make coding more python3-ish' comment per test suite convention
- Fix load_fixtures to select empty fixture via filename='empty' parameter
- Update purge test to assert correct post-fix behavior (no vif None commands)
- Apply vyos_vlan purge fix to allow tests to pass (will be superseded by T8520 on merge)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>