test-network: reconfigure interface cleanly to drop previous DHCP lease and friends

test-network: reconfigure interface cleanly to drop previous DHCP lease and friends

Follow-up for 451c2baf30f50b95d73e648058c7c2348dbf0c31.

With the commits, reloading .network files does not release previously
acquired DHCP lease and friends if possible.

On graceful reconfigure triggered by the reload, the interface may
acquire a new DHCPv4 lease earlier than DHCPv6 lease. In that case,
the check will fail as it is done with the new DHCPv4 lease and old
DHCPv6 lease, which does not contain any IPv6 DNS servers or so.
So, when switching from no -> yes, we need to wait a new lease with DNS
servers or so. To achieve that, we need to clean reconfigure the interface.