new: test: Add AnsInstance as the ans counterpart of NamedInstance

Tests interacting with mock ans servers had to hardcode their IP
addresses and open ans.run directly, while named instances already
had the NamedInstance abstraction with `.ip`, `.log` and the
watch_log_*() helpers.  Factor the parts of NamedInstance that are
not named-specific into a ServerInstance base class and add an
AnsInstance subclass for ans servers, exposed through the `servers`
fixture and new ans1-ans11 convenience fixtures.

Assisted-by: Claude:claude-fable-5
Merge branch 'nicki/pytest-ans-instance' into 'main'

See merge request isc-projects/bind9!12241