Fix, add, remove IPv6 tests for DHCPv6, API

According to new functionality in review 129144
Also coverage for bugs 1382076, 1330826
Expected fails:
Test test_dhcp_stateless_router fails because 1382076
Test test_dhcpv6_stateless_two_subnets fails because 1358709
ra_mode and address_mode are supposed to be the same.