IPv6 subnets now support a boolean attribute called slac.
If a request for a subnet contains both a slac field and an enable_dhcp
field, only the slac one will be used. If no slac field is found for an
IPv6 subnet, the field enable_dhcp will be used instead.

IPv4 subnets silently ignore any slac field if present and only use
enable_dhcp.

This commit adds the relevant tests in tests/subnets.py too.