chg: test: Add NSEC3 answer correctness test to dnssec_py

Rewrite nsec3_answer/tests_nsec3.py as dnssec_py/tests_nsec3_answer.py
using the isctest.zone helpers for zone setup. ns1 (auth) and ns2
(resolver) were renumbered to ns2 and ns9 respectively to fit the
existing dnssec_py server infrastructure.

Assisted-by: Claude:claude-opus-4-8
Merge branch 'nicki/pytest-dnssec-py-nsec3-answer' into 'main'

See merge request isc-projects/bind9!12209