tests: net: dhcpv6: Add tests

Add tests for the DHCPv6 library. The tests verify that:
* message format is valid for the outgoing messages
* incoming messages are only processed in respective states
* processing through standard DHCPv6 transactions is correct
  (Solicit/Confirm/Renew/Rebind)

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>