When IPv6 range is be enabled in testnodes, `_ipv6_address` does not
contain a single IPv6 address shared by all partitions anymore, but
instead the IPv6 range from which each partition will obtain its own
IPv6 address. Instead there is now `computer_partition_ipv6_address`
which contains the IPv6 address of the main partition.

This change blindly replaces all occurences of `_ipv6_address` with
`computer_partition_ipv6_address`, since that will fix most simple
cases. The remaining cases, such as tests with instance trees that
use multiple partitions, can be investigated and fixed individually.

/reviewed-on nexedi/slapos!350