ipv4:dhcp: add new tests for client-id "none" and default
A new special value "none" is being introduced for ipv4.dhcp-client-id in NM 1.45.5. It works with dhclient and internal client. Add new tests to verify its behaviour.
Also, add 2 new tests for the default value of ipv4.dhcp-client-id when using dhclient. The default behaviour is to use the client-id if it's defined in dhclient.conf, or don't send any client-id otherwise.