[9.20] new: test: IPv6 case to isc_netaddr_masktoprefixlen tests

Unit test for isc_netaddr_masktoprefixlen were missing IPv6 mask cases.
Add those and few other IPv4 cases. Also, the test is refactored in
order to make it easy to add new cases.

Backport of MR !10328

Merge branch 'backport-colin-netaddrmasktest-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10332