T8516: restore _state_deleted docstring, align test docstring and fixture
- restore the _state_deleted docstring lost in the L2-safeguard change, now documenting the address-only delete semantics
- overridden test docstring said "delete interface stanzas" but the module now emits address-only deletes; align wording
- quote loopback interface name in fixture for consistency with vyos_interfaces_config.cfg